Haru - Absconditus

If you don't have her yet but want to try her out, she is available as a Trial Operative in Story Mission 11-12 (Legacy - Containment Zone - Endless Snow // Fogbound Dream)

Absconditus deals ok damage, and can hit multiple enemies, but she is limited to melee range, doesn't have any way to group up enemies for her AoE melee slashes, and constantly being in melee range can give her quite poor survivability.
Haru has also been left behind by powercreep, and having only mediocre damage plus her melee range makes her a very poor choice in comparison to newer operatives.

Overall

Role: Main DPS - Hybrid
Deals Kinetic Damage
Weapon Type: Pistol
Deals both ballistic and skill damage
Mobility is very good

How to use Haru - Absconditus

If Haru's ult is available:

Use her ult
Then spam her standard skill to deal melee damage in front of her

Otherwise:

Spam Haru's standard skill to deal melee damage in front of her.
Shoot the enemy to recover your S-Energy has recovered and reduce the cooldown of your standard skill

Repeat

How to build Haru - Absconditus

Weapon: 16-Psyche / Star Ocean / Yojimbo
Weapon Parts: Max ATK
Logistics Squad: Akitsu
+10.0% ATK
+106 Alignment Index
(or +19.3 Skill Haste)
+8.5% Kinetic Damage
Best teammates:
Vidya - Celestial Swan
Marian - Aquila
Eatchel - The Cub
Siris - Freefin / Marian - Riptide

Passive Effect
Each shot Haru lands with her pistol restores 2 S-Energy and reduces her standard skill cooldown by 2s.

After attacking a target, Haru's Deiwos allows her to deal automatically deal True DMG to the target (i.e. instakilling them) if they have less than 40% of Haru's ATK.
(40% base, plus an extra 10% per 100 Alignment Index.)
As usual for instakill effects, the trigger threshold is so low it's completely useless against bosses.

Standard Skill
Absconditus slashes in front of her, hitting up to 5 enemies in front of her.
This skill has a short-ish cooldown and low S-Energy cost, and uses a charge mechanic, storing up to 5 charges.

Since hits with her pistol decreases the cooldown of this skill, you can spam this skill, magdump and reload, and then repeat.

With its second Neuronic effect, defeating an enemy with this skill will restore 1 charge and mean it doesn't use S-Energy, which can be a nice S-Energy boost.
If Haru can keep killing, she can keep spamming this, potentially making her very strong against groups.

Ultimate Skill
Absconditus slashes in front of her to deal Kinetic damage.
Haru gains 50 Victory Points.
Each second 1 point is consumed. Her standard skill costs no S-Energy, and has a shorter cooldown.
Each use of her standard skill consumes an extra 2 Victory Points.
With her standard skills Neuronic buff, there is a change her standard skill doesn't use Victory Points.
Whilst she has Victory Points, Haru doesn't recover U-Energy from shooting.
You can use her ult again to consume 15 Victory Points to deal strong kinetic damage in front of Haru.
The cooldown for her ult starts when she runs out of Victory Points.

Support Skill
Absconditus slashes in front of her to deal Kinetic damage.

Like most main-dps operatives, you really don't want to use Absconditus as a support.

Weapon

16-Psyche
Her 5* signature weapon.
+Kinetic DMG, with +ATK and +Skill DMG triggered by defeating enemies or using her standard skill.
Provided you are spamming Haru's standard skill this is easy to max.
Star Ocean
Inflicts a DoT after a skill hits an enemy, which deals damage ever 0.5s.
It doesn't stack - Repeated hits simply reset the duration..
Star Ocean gives less buffs than 16-Psyche, but does actually give Haru more damage.
Star Ocean is also on the standard weapon banner, and is thus much easier to get.

Both 16-Psyche and Star Ocean are good weapons for Haru, and are somewhat comparable in their performance.

Yojimbo
+25.2-42% Skill DMG.
Her best 4* weapon.
Nexus
+6% Standard Skill DMG per second, up to 10 stacks.
Deals Chaos damage instead of Kinetic, but is otherwise better than Yojimbo due to its stronger buff.
Wild Wasp Stinger
Gives +6-10% ATK for 5s (up to 4 stacks) if repeatedly using the same skill.
Her signature 4* weapon, and the games recommended weapon for Absconditus, but isn't actually that good.
Objectively worse than Yojimbo, as well as being more clunky to use as its buff isn't permanent and requires building up over time.

For weapon Parts, Haru wants maximum ATK.

Logistics

Good logistics squads:

Akitsu
Akitsu gives her 1 stack every time she uses her standard skill, 10 when she uses her ult, and 5 upon defeating an enemy.
At 15 stacks she gets +25% ATK for 10s.
At 30 stacks she gets +20% Kinetic DMG for 10s.
All stacks are cleared when she reaches 30.
This provides a good ATK and Kinetic DMG buff, provided she can keep spamming her standard skill.

Twilight
After a skill damages an enemy, Twilight reduces the targets resistance to that damage type by 16% for 5s.
Since Hary spams her standard skill a lot, Twilight is easy to maintain for her.
Thebes
Gives +2% ATK for 5s after hitting an enemy with a skill. Max 20 stacks.
Haru spams her standard skill a lot, making it easy for her to build stacks of this.

Which is best generally depends on what other supports Haru has - with Vidya - Celestial Swan Twilight is much less effective as Celestial Swan already gives so much ResPen.
Akitsu and Thebes both work well. If defeating enemies often then Akitsu is better. Otherwise Akitsu is a little better, but not by too much.
Thebes is easy to get though as it's one of the standard squads, and does work very well.

For the special talent, Haru wants +8.5% Kinetic Damage
For the basic talents, Haru wants +10.0% ATK.

For the second basic talent:
Alignment Index increases the threshold of her instakill.
With three max-roll Alignment Index stats, the threshold will increase from 40% to 71.8% of Haru's ATK.
(So increasing from maybe 2,000 HP to around 3,500 HP.)
This is a decent increase, but Haru deals enough damage and can spam her standard skill enough that this doesn't really help her that much.
It does have some use against trash mobs, but with enough investment Haru doesn't really struggle too much with those.

Haru can go for Skill Haste, but with her ult active that's mostly useless.
Without her ult active, Haru can simply shoot enemies to recover S-Energy and reduce the cooldown on her standard skill, so she doesn't really need Skill Haste.
Even if you did get Skill Haste, she can start to run into S-Energy issues over very long fights if she doesn't shoot enemies much.

No second stat really helps her that much Alignment Index and Skill Haste aren't great, but they're the best she's got.
Alignment Index makes her a bit better for regular gameplay as it helps her against trash mobs, whereas Skill Haste is a bit better against bosses.
Niether are massively useful though. I'd go for Alignment Index myself, but Skill Haste is decent.

Manifestation

With M1 Haru can use her ult for free every 10 minutes.
This basically means that you get one free ult per mission.
You can just save her ult for the final boss to quickly wail on them. It works pretty well.
M2 Increases Haru's ATK by 5% for 5s after using a skill, with repeated triggers resetting the duration of all stacks.
Give how often Haru uses her skill, this is basically +25% ATK, which is very nice.
M3 gives Haru Iron Body when using a skill
Iron Body gives +20% damage dealt and -20% damage received.
Which is very nice.
M4 increases the level of her standard skill by 1, giving a cd reduction of 20%, and a 20% reduction in S-Energy cost.
M4 allows Haru to spam her standard skill more often, which is great.
With M5, when Haru has any Victory Points from her ult, Haru deals more damage the lower the targets HP, up to 30%.
This doesn't average to that much, but it is still a damage increase nonetheless.

If you want efficiency (good damage without massive investment), then M3 is a good stopping point.

Which teammates to use with Absconditus?

Haru deals mostly skill damage, with a little bit of ballistic damage.

Her ballistic damage is so low that it's not worth investing into buffs for it.
Similarly, her RoF is so low that per-shot bonuses from Fritia - Little Sunshine or Tess - Phantasia are wasted on Haru.

Haru wants skill-damage buffs, ATK buffs, and to debuff the enemies resistances and DEF:

Vidya - Celestial Swan (and other resistance shredders)
Celestial Swan and other resistance shredders (Acacia - Kaguya, etc.) that reduce the enemies resistance help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off element to the main DPS.
Marian - Aquila
Aquila provides very strong generic damage buffs, which makes her an excellent support.
Aquila is on-element to Absconditus (Aquila constantly deals Kinetic damage).
Siris - Freefin
Freefin provides strong skill damage buffs, although Absconditus can't make use of Freefin's Wavehunt thermal skill damage buff.
Marian - Riptide
Haru spams skills, so Haru can build up Riptide's shots very quickly.
Note that Riptide is off element to Haru (Riptide deals chaos damage)
Eatchel - The Cub
Eatchel provides some nice ATK buffs, and Haru spending all of her time in combat in melee range of enemies means that Eatchel's healing is very nice for her.
Chenxing - The Observer
Chenxing buffs skill damage, and provides a little healing as well, both of which Haru tends to want.
Enya - Exuvia
Haru does both use skills a lot but also shoot a bit.
Haru's low average RoF means that Enya isn't ideal, but Haru can still make use of her if you don't have a better support.
Exuvia is off-element to Absconditus though.
Tess - The Magician
Tess can allow Haru to ult very quickly, but once Haru has her M1 she can use her ult for free once per mission anyway, which puts it onto a 60s cooldown.
If you're up against a boss, then after that first ult you are likely not going to use her ult again due to the long cooldown, even with Tess.
As such, Tess is good if Haru doesn't have her M1 yet, but Tess isn't overly strong once Haru does get her M1.