Marian - Riptide

Marian - Riptide is not currently available as a Trial Operative in any story mission, so if you don't have her the only way to try her is to wait for her banner so she has a trial.

Riptide is a Chaos Damage Skill Sub-DPS who both deals decent damage and also provides Skill DMG buffs to the team.
Riptide does deal good damage, and the Skill DMG buffs make her quite easy to fit alongside most skill dps characters, although Riptide does want to swap in quite often, which can be a problem for some dps-ers, although being one of the games few pure-skill dmg buffers means Riptide is a popular support for skill dmg characters.
Riptide doesn't work very well as a main dps, but also doesn't work well as a purely off-field support either - she needs to swap in in order to work properly.

The Basics

Role: Sub-DPS
Deals Chaos Damage
Weapon Type: Shotgun
Deals entirely skill damage
Mobility is about standard

How to use Marian - Riptide

Use your Main-DPS
Riptide gains Tidesong passively, as well as whenever your main-dps operative uses a skill.
Tidesong gives Riptide special shotgun shells, up to a max of 4
When Riptide has maxed her Tidesong shells, use her support skill to swap her in and magdump
Swap your main dps back in

Repeat

How to build Marian - Riptide

Weapon: Abyssal Nymph / Opal Prism
Weapon Parts: Max ATK
Logistics Squad: Aileron
+10.0% ATK
+106 Alignment Index
+8.5% Chaos Damage
Best teammates for Riptide to Support:
Nerida - Styx Envoy
Haru - Raikou
Cherno - Enigma
All other Skill-damage operatives

Standard Skill

Marian loads 1 Kraken round into her shotgun.
Kraken rounds have basically infinite range, have a very wide hitbox (making aiming quite easy), and do vastly higher damage than a regular shot.
(With enough investment potentially over 10x the damage of a regular shotgun round.)

Note that Kraken Rounds count as skill dmg, not shooting.

Ultimate Skill

Riptide basically throws what is effectively a grenade in front of her to deal AoE Chaos damage.

Riptide's ult is very weak, and you're better off using the U-Energy for your other squad members.

Support Skill

If Riptide is off-field then whenever a standard skill is used [by any operative] Riptide gains Tidesong - this charges up her Kraken magazine.
Riptide gains 3 Tideside per second and an extra 7 Tidesong per standard skill usage. Each Kraken round needs 25 Tidesong, for 100 Tidesong for a full mag of 4 rounds.

Once Riptide has accumulated enough Tidesong to have at least one Kraken round, you can use her Support Skill to swap Marian in.
When swapped in, Riptide will have her Kraken mag loaded and ready to go, so you can just instantly click to start magdumping all her Kraken rounds.

Riptide is best played as a sub-DPS, waiting until her Tidesong is maxed and she has 4 Kraken rounds, swapping in to shoot all her Kraken rounds, then leaving the field to let her Tidesong recharge.

Note that Tidesong counts as a special magazine, and hence can't be used alongside the special mags from Fenny - Starshine.
Note further that her Support Skill loads the Kraken Magazine from her Standard Skill, so if you see some of Riptide's buffs say they buff her standard skill that's why - it's because her support skill is an extension of her standard skill - Her Support Skill basically just swaps her in and procs her Standard Skill.

Weapons

Abyssal Nymph
Her 5*, and her best weapon, giving her lots of Skill DMG and Chaos ResPen.
This also gives a Standard Skill DMG buff to th deployed operatives too, which is nice, although it's not a massive bonus.
Opal Prism
Her signature 4*, which gives her a very large Skill DMG boost.

If you don't have either of her signature weapons, then use Neon Tube - it gives a nice damage dealt bonus when HP is above 50%, which is pretty easy for Riptide to maintain.

For Weapon Parts Riptide wants max ATK.

Logistics

Use Aileron squad - it was designed specifically for her and is her best choice.
Note that when Aileron was first launched, it worked with any operative, allowing any characters to give the entire team the +30% Skill DMG.
This has long since been patched and Aileron now only works with Riptide, but if you see old posts mentioning Aileron for other operatives, that's why.

For stats Riptide wants +10.0% ATK and +106 Alignment Index.
For the Special talent, Riptide wants +8.5% Chaos Damage

Just max damage, basically.
ATK increases her damage, as does Chaos damage.
Alignment Index gives the entire squad a Standard Skill DMG boost, which is great.
Since Riptide is there to provide skill dmg buffs to your main dps, Alignment Index is debatably her best stat - better than ATK, as all operatives benefit from her Alignment Index.

As a skill-dps, crit mods are useless to Marian, as skills can't crit.
Her Tidesong isn't affected by S/U-Energy regen, so those are useless to her as well.

Manifestation

M1 increases the RoF of her Kraken Mag, allowing for more shorter rotations and thus more dps.
It also gives a 10% Final DMG boost to Kraken Shells, which is very nice.
M2 gives the entire squad a 20% Skill DMG increase for 8s whenever the number of Kraken Shells Riptide has changes.
If using Riptide properly as a Sub-DPS this is basically a permanent +20% Skill DMG bonus for the entire team, which is very nice.
M3 gives Riptide +20% Tidesong Acquisition Rate if equipped with a Chaos weapon (i.e. hopefully always).
Like M1, this allows for more shorter rotations and thus more dps.
M4 gives her Standard Skill +1 level, which basically just gives her Kraken Shells more damage, which is very nice.
M5 gives Kraken Shells 20% DEFpen if Riptide is equipped with a Chaos weapon (i.e. hopefully always), which is a nice damage buff.

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

Good Teammates for Riptide

Riptide deals Chaos damage and buffs skill damage, so she is fantastic for supporting operatives that deal mostly chaos skill damage:

Nerida - Styx Envoy
Haru - Raikou
Cherno - Enigma

Riptide's Skill DMG buffs mean that she works well with basically any character that deals lots of Skill DMG:
Clarina - Crimson Moon
Enya - Tempest
Fenny - Pureheart
Katya - Dawnwing
Chenxing - Jade Arc
Fritia - Hush
etc...
If an operative deals mostly skill DMG, then Riptide will work well with them.

Riptide's M2 Skill DMG buff only lasting 8s means that she can be tricky to fit into the rotations of certain characters (notably Fritia - Hush and Fenny - Pureheart) however she will work and is still good regardless of how good her synergy is with the main-dps' rotation.

A few notes:

Riptide deals Chaos damage.
This means that if you are also using Vidya - Celestial Swan alongside Riptide then you will want your main DPS deal Chaos damage, in order to make use of Celestial Swan's strong debuffing. If your main dps doesn't deal Chaos-damage, then avoid using Riptide alongside Celestial Swan.
Nerida - Styx Envoy really really wants Riptide on her team - Styx Envoy was designed around having Riptide as her Sub-DPS, and Styx Envoy performs much worse if she doesn't have Riptide as her Sub-DPS.
I recommend you don't play Styx Envoy unless you also have Riptide to pair with her.
The very long duration of Fritia - Hush' ults means that Riptide's M2 bonus might not affect the latter parts of Hush' ults, which are the most damaging parts.
Riptide is thus not the best support for Hush. She's still good, but there are better options.
You can time it so that Hush' ult does have Marian gain Tidesong part-way through and thus works as she should, but that can be difficult and/or tedious to do.