Fenny - Lionheart

Lionheart is available as a Trial Operative in Story Mission 1-2.
You can also try her out as a Trial Operative in Star Master at max level and with a good weapon, great logistics, max Neuronics/Deiwos, and max Manifestations.

Fenny - Lionheart is a pretty simple character to play - just spam her standard skill and shoot things with her shotgun, which also makes her quite fun to play, and her naturally high ATK stat means she deals quite a lof of damage, but whilst she's one of the best 4* operatives for damage, being a 4* operative means she still doesn't compete with the 5* operatives and her lack of good support functionality means she isn't useful at endgame, and whilst she is decently bulky constantly being in melee range of enemies can lead to poor survivability. Fennys M5 buff means she can work as a support, but not recommended.

Overall

Role: Early-Game: Main DPS
Late-Game: Parts Break w/ weak +ATK buff
Deals Kinetic Damage
Deals almost entirely ballistic damage
Mobility is very good

How to use Fenny - Lionheart

As a main DPS:
1. Spam standard skill off-cooldown
2. Otherwise, just shoot everything that moves

As a support:
1. Spam her Support Skill off-cooldown.

How to build Fenny - Lionheart

Weapon:
As a DPS: Tiny Grains
(Maritime Scout also good)
As a support: Lapis Lazuli.
Weapon Parts: Max ATK
Logistics Squad: Amano-Iwato
Basic Stats: +10.0% ATK
+106 Alignment Index
Special Stat: +8.5% Kinetic Damage
Best squadmates [as a main dps]:
Fritia - Little Sunshine
Yao - Quiet Quitter

Attack
Fenny's standard attack is a regular shotgun attack.
Being a shotgun, it has a short range but can break parts easily.

Standard Skill
Lionheart's Standard Skill causes her to charge forward, shooting nearby enemies when she arrives at her target.
With her Neuronic effect, this also grants Fenny a shield for 5 seconds if she has a Kinetic shotgun, which is a nice survivability bonus for her.

By-default using her standard skill costs 1 shot, which is quickly and automatically reloaded after the skill completes.
Getting her to Manifestation 2 removes this 1-bullet requirement.

This skill deals 33% of Fenny's ATK, 8 times.
Since shotguns typically fire around 8 pellets and have around 30-35% Compatibility, this means her skill deals very roughly about the same damage as one shot of her shotgun.
... but this skill also grants shield, gives her mobility, and with M3 increases the damage of her next shot by 50%, so this skill is better than just shooting normally.

Basically just spam her standard skill as much as you can.

Ultimate Skill
Lionheart's Ultimate Skill causes her to fire in a line in front of her, dealing damage 7 times.
Hitting a targeted repeatedly causes subsequent hits to deal 50% less damage each time, meaning there's not that much point trying to hit a target lots of times - even 2-3x is very good and about as much as you'd want/need.

With her Neuronic buff, this skill does heal for 5% of the damage dealt.
This doesn't translate to a lot though, and if you want healing you're better off using a dedicated healer support operative like Eatchel or Quiet Quitter.
It is something though, and combined with the shield from her standard skill it can make Lionheart one of the more bulky 4* operatives.

Generally speaking, I'd generally recommend that you avoid using Lionheart's ultimate skill, and instead use your squads U-Energy for the ult of another operative in your squad.

Support Skill
Lionheart's Support Skill deals Kinetic damage in front of her, and also recovers some bullets.
With her Neuronic buff, each target hit reduces the cooldown by 2 seconds, up to 10s max reduction.

With her Manifestation 5 effect, Lionheart's Support Skill also gives a 15% ATK buff for 5s.

Manifestation

M1 resets the cooldown on Lionhearts Standard Skill when she defeats an enemy, and the next usage doesn't cost S-Energy.
The S-Energy cost reduction is a minor benefit, but the cooldown reset is a nice QoL change, albeit one that doesn't really affect her damage that much.
M2 removes the 1 bullet cost of her standard skill.
M3 causes Lionheart's next shot to fire 50% more pellets after using her standard skill
M4 increases the level of Lionheart's standard skill by 1 and increases its damage
Given how long this skills cooldown is, this isn't a large damage buff.
M5 causes Lionheart's support skill to also give a 15% ATK buff for 5 seconds upon use.
5s is a short duration, so the uptime of the buff is low.

Being a 4* operative, you'll get all her Manifestation buffs eventually.

Lionheart's Manifestations mostly buff her standard skill, which is good, as her ultimate isn't very good.
Given that Lionheart is one of the stronger operatives you have access to in the beginning of the game, buffing her standard skill does work out well for her as that increases her mobility and survivability, however the long cooldown means that none of her Manifestations actually make that much difference to her dps.

M5 Fenny with Lapis Lazuli and a support logistics squad can work as a support. She's not as good as any 5* support operative, but she can work, although by the time you get her to M5 you will have 5* operatives that perform much better as supports.
By the time you get Lionheart to M5, you will have better 5* operatives though.
(For reference, despite being what I'd consider an experienced player and making this entire site on the games mechanics and operatives, I still don't have Lionheart at M5.)
(I am the unlucky-est player in all of RNG and gacha history though...)
Even if you only have 4* operatives, you still won't want to use Lionheart as a support...

If you are considering using her as a support operative then you probably don't have the investment needed to make her really work as a support operative, and if you did have the investment to make her a decent support, then you very probably have some 5* operatives you could use instead who will perform much better and are more worth the investment.
As such, it isn't worth trying to make Lionheart into a support to make use of her M5 bonus, but it is do-able.

Weapon

As a DPS:

Tiny Grains
A 5* Kinetic Shotgun.
The requirement to take damage in order to proc its buff isn't ideal, although since Lionheart charges at enemies so much she usually doesn't find it that to hard to proc.
Even without the buff, Tiny Grains is strong and it is Lionheart's strongest weapon.
Given that Lionheart is a 4* operative though, you're better off giving your 5* weapons to your 5* operatives if you have any that can use it.
Maritime Scout
A difficult-to-get weapon, but it's Ballistic, Standard Skill, and Ultimate Skill DMG buffs are very strong, and make it Lionheart's best 4* weapon, and second best to Tiny Grains.
If you aren't proc'ing Tiny Grains buff by being injured constantly, then Maritime Scout performs somewhat similarly, with the only real difference between the two being base damage - Tiny Grains has higher base damage (811 vs 645, or +25% more).
Precious Baby
Fenny does make a shield quite frequently due to her standard skill, so Precious Baby's 24-40% extra shield construction efficiency can be a useful stat for her to improve her survivability. Extra survivability is usually not needed though, and it is a minor buff.

The game recommends Mechanized Cop for Fenny, and whilst it is a Kinetic Shotgun and thus does work well for her, its 1-shell capacity means that you have to reload after every shot, which gets real annoying real fast, especially as it doesn't have a particularly fast reload speed.
Its very high Compatibility stat means it does deal more damage per shot (about two-thirds more damage than that of a regular 4* shotgun), but its constant reloading means its dps isn't actually that great - It deals around 1% more actual dps than Full Alert, and around 2% more than Precious Baby. Factoring in it's effect, Mechanized cop on-average deals less damage than Maritime Scout unless you are killing enemies very frequently to proc Mechanized Cops effect.
Mechanized Cop is, technically speaking, a good shotgun, and it's perfectly fine to use it but the advantage it has over other 4* shotguns is tiny, and given how annoying it is to play I don't recommend it - just use Maritime Scout or Precious Baby.

If you don't have Lionheart's first Neuronic, then Lionheart doesn't gain any benefit from a Kinetic weapon specifically, and as such if she doesn't have that Neuronic then she can use any shotgun just fine, in which case other shotguns like Neon Tube and Discordance are just fine.

As a Support:

Lapis Lazuli
When the equipping operative uses their support skill, Lapis Lazuli grants +ATK to the equipping operative for 15s.
Lionheart's Support Skill has a 25s cooldown, but if you have her Neuronic then hitting enemies can reduce its cooldown, and Skill Haste on her logistics can reduce it further, which can allow Lapis Lazuli to have 100% or nearly-100% uptime.
Rock Python
The ATK buff if the target is at full HP can be a nice bonus, if you can proc it.
Whilst it's not a large bonus, combined with Fenny's M5 bonus and a support-based logistics squad then Fenny can give out several buffs to her team, which add up to a decent amount, although it's not as good as Lapis Lazuli, nor as consistent.

For weapon Parts, just go with ATK.

Logistics

Lionheart is a 4* operative and is completely outclassed by 5* operatives.
As such, please don't waste logistics re-rolls on Lionheart.
Similarly, if you have the resources to get Lionheart a new logistics set, then you should put that investment towards your 5* operatives if you have any.

As a main DPS Lionheart can use:

Amano-Iwato
Increases ADS Ballistic DMG by 10% after a crit. Max 5 Stacks. A great choice if you are ADS'ing with Lionheart.
If you only hipfire and if you don't ADS then it's obviously terrible, although given Lionheart's poor options for other squads, using Amano-Iwato with ADS is Lionheart's best way of being used as a main DPS.
Mizuho
When shooting continuously, each shot increases Ballistic DMG by 3%. Max 20 stacks.
Lionheart's small magazine size mean the bonus she gains from Mizuho is limited, but it's still one of her better options for hipfire shooting.
Mingyi
When dealing Electrical DMG, increases ATK of the equipping operative by 1% for 3 sec. Max 45 stacks. Repeated triggers will reset the duration.
Still less total damage increase than Amano-Iwato, but again it doesn't need ADS, and you can stop part-way through magdumping to use a skill, which is nice.
The disadvantage is that Lionheart needs an Electrical Shotgun, which means she doesn't get the Neuronic buff to her standard skill that gives her a shield.
This is not the worst thing ever, but it is notable as a survivability loss.
Whilst I recommend you use a Kinetic Shotgun to gain the benefit of Lionheart's Neuronic effect as well as being her best choice for dps, if you did want to hipfire with her shotgun and not ADS, then Mingyi with an electrical shotgun (Discordance or Sunny Payback) is her best choice.

For stats: If Lionheart is a main DPS, she wants +10.0% ATK and +106 Alignment Index.
For the Special talent, you want 8.5% Kinetic Damage, or Electrical Damage if using Mingyi and and Electric shotgun

As a support:

Amarna
Increases Ballistic DMG by 30% for 15s after using her support skill.
This is quite a nice buff, and whilst Lionheart can't make much use of the "Aten's Sight" part of the logistics squad, +30% ballistic DMG is still decent.
Twilight
After dealing DMG with an Active Skill or Auxiliary Unit, the target's Resistance to that type of DMG decreases by 16% for 5 sec.
Lionheart's support skill has a long cooldown, but if the main dps she is supporting is quite bursty (e.g. Tempest) then Twilight can be a very good squad for them.
Similarly, for non-ballistic operatives, Amarna is basically useless, so Twilight is likely Lionheart's best support squad.
Since Lionhearts Support Skill deals Kinetic damage, Twilight will debuff kinetic resistance, meaning it is only useful to support dps operatives that deal kinetic damage.

If Lionheart is a support operative, she wants ATK and Skill Haste as her stats.
For the Special talent, she wants 8.5% Kinetic Damage
Again though, Lionheart is a weak support, and takes a lot of investment to work as a support at all, so she isn't really worth using as a support, and certainly not worth wasting re-rolls on.

Good Teammates for Fenny - Lionheart

If using Lionheart as a main DPS, then all of the usual supports work well: Celestial Swan, Kaguya, The Cub, Nightglow, Shadow Ka, Phantasia, etc.
Lionheart's best supports are Celestial Swan and Nightglow/Phantasia, however since Nightglow herself does out-dps Lionheart then if you actually have such 5* operatives you should use another main dps instead of Lionheart.

If you only have access to 4* operatives to support Lionheart, then consider:

[Healer] Yao - Quiet Quitter
Yao creates a drone that provides ample PBAoE healing, which whilst not overly useful in endgame content, it does make Lionheart much nicer to play.
For endgame content QQ is objectively worse than the 5* Eatchel - The Cub though.
Fritia - Little Sunshine
Little Sunshine can give an aptitude buff that gives per-shot Thermal damage.
Fritia's Aptitude buff grants extra damage per each pellet of Fenny's shotgun shells, meaning it triggers 8 times per shot.
This means that Little Sunshine adds a lot of extra damage to Lionheart.
Note that Lionheart's standard skill is not a shot and hence doesn't benefit from Little Sunshine's extra damage.
Acacia - [Redacted]
Acacia's slow is a useful QoL and survivability buff, albeit minor.

If only using 4* operatives, Fenny's best supports are Little Sunshine and Quiet Quitter.