Sangonomiya Kokomi (珊瑚宮心海) of Watatsumi Island is a healer released in Version 2.1. In accordance with the 0 sum mentality of certain USA elites: Kokomi is better than Ayaka and does more damage.
The name Sango as well as her C6 Sango Isshin (Shanhu Yixin) is an early example of interference in corporate affairs. Other characters such as Kaveh and Mika are later examples of interference in corporate affairs, using voice actors to combine the names of coworkers in order to influence office politics. This suggests Genshin is trying to build a clique of political allies around Sango Isshin. But this hardly matters now since all the relevant people hu, wei have been fired already for attacking the wrong people.
As for the name Kokomi (Xinhai), that can also refer to several different things, from a mass murderer to a certain revolution.
The name shanhu reminds one of the name of a place where a corrupt politician was executed. Thinking about that, it gives some extra meaning to why Raiden Shogun and Kokomi were released in the same patch: it is pairing the executioner with the executed. The hugong reminds one of the great grand daddy of the hu clan from ancient times. It is to remind everyone who their father is and whose genes they carry.
Her healing and her burst damage all scale with HP. Her jellyfish (E) heals on field characters within an AoE. It has no ICD on the hydro application.
Her burst (Q) refreshes her E's duration and allows her NA to heal the entire party (including off field characters). Moreover her NA gain bonus damage based on her HP and healing bonus. She can also walk on water during her 10 second burst. The energy cost is 70, same as Xiao and Itto.
With one of her passives, Kokomi gets -100% CRIT Rate (so initial of \(5-100=-95\)%) but increased Healing Bonus. The inability to crit will turn out to be her Achilles heel when discussing her dps.
For a burst damage build, talent priority is Q = AA > E. For a pure off field healing build, priority is E > Q > AA.
For a burst damage dealer (and for more damage in general), the Ocean Clam (released in Version 2.3) 4 piece is her bis. If you only care about her off field healing (her E) and aim to use her as a buffer (harmony unit), then 4 piece Tenacity is also viable. Other mixed sets such as 2 piece Tenacity / 2 piece Clam / 2 piece Heart of Depth / 2 piece Nymph can also work. Noblesse Oblige can also work.
HP sands, HP/Hydro DMG goblet, Healing bonus circlet. Substats: HP, ER. This will maximize her healing and damage (since it scales with HP and healing bonus). Note she gains Hydro damage bonus upon ascension.
For the 4 piece Clam set, most of her damage will be from the bubble pop on the 4 piece set (which does more damage the more you heal, up to 30k before Physical RES is factored), so you still want to maximize her healing.
One of her best weapons is R5 Thrilling Tales of Dragon Slayer for HP stats and ATK buff. Another option is Sacrificial Fragments (usually for Nilou bloom or for resetting E).
Sacfrag does not change the fact that you still have to manually switch into Kokomi within 12s to reset the E given the E only lasts 12s at a time (without sacfrag or Q reset). By comparison, characters like Nahida, Raiden, and even C2 Dehya have off-field E app that lasts 18+ seconds. While Kokomi's 12s off-field app may be enough for quickswap teams (such as Alhaitham), it is still not long enough for several carries (such as hyperbloom Cyno who has an 18s on-field burst).
Nilou C1 may be the closest such substitute but Nilou's off-field E app does no damage (compared to Nahida E or Raiden E hyperbloom, which are available at C0). For these reasons, there is still room to create a better hydro applicator than Kokomi (and Nilou): one that has 100% uptime, lasts 18+ seconds off-field, does not require Q reset, does decent damage (and preferably at C0).
Kokomi is typically used in freeze teams, electro-charged teams, the meme Kokomon team, bloom teams, or basically any team that needs healing (like against the rifthounds or other bleed mechanics).
Even though her E has no ICD so she can technically be used in vape teams (she can keep up with Xiangling's Pyronado), for vape teams usually Childe or Xingqiu are preferred as Hydro applicators.
Note in freeze teams, usually Mona or Xingqiu are run for off field hydro application, whereas Kokomi takes field time away from your Cryo and Anemo units.
Electro-charged teams with Beidou and Raiden are viable. In such teams, even a Cryo unit (like Rosaria, Kaeya, or possibly Shenhe) can be run as the Superconduct reaction will lower enemy's Physical RES and make Kokomi's bubble do more damage.
The Kokomon team consisting of Kokomi, Xiangling, Fischl, Sucrose (and their pokemon) is an example of a team that can trigger 4 way elemental reactions. Basically the hydro and electro element can coexist on one enemy. Then the pyro element can be applied to another enemy. Then using Sucrose as a driver to swirl these elements can create a big explosion where the enemies will simultaneously be applied with hydro, electro, pyro, anemo (from swirl).
Kokomi is also used for bloom, hyperbloom, burgeon. This is perhaps the best way to use her as things like hyperbloom have high dps. Example teammates include Dehya (burgeon), Alhaitham, Nahida, Nilou, Thoma, Raiden (hyper/rainbow), Kuki, Yae, etc.
There are two parts to her dps: her Q dps and the Ocean Clam dps. Assume 30k hp and lv10 NA, E, Q. Assume 4p Clam with \(46.6+28.8=75.4\% \) hydro dmg bonus and \(25+15=40\% \) healing bonus.
Also for sake of argument, suppose she has 2k atk and uses Bennett. In reality, for the teams Kokomi plays in, neither condition may be true, and as a result the dps below (specifically from her ATK scaling parts) below will be less.
At lv10, her NA gets an additive bonus of 12.77% hp. With her passive 15% of her healing bonus is converted into extra hp scaling. So for 40% healing bonus, she gets an additional 6% hp scaling, giving an additive bonus of 18.77% hp for her NA. This is equivalent to 187.7% atk scaling per NA.
Suppose she uses her 3-hit NA combo 5 times within the Q's 10s interval (this becomes easier to achieve with atk spd buffs like Mika's). The burst has a cd of 18s. Her Q does an initial hit of 18.75% hp or around 187.5% atk scaling. The mv is $$x_b = (187.5+5*(123.08+110.77+169.75+3*187.7))*1.754/18 = 489.3$$
This is actually higher than Ayaka's mv. Moreover, the scaling of an individual NA goes up to \(169.75+187.7=357.45\) which is considered a nuke for a hydro attack (given this can vape). Moreover, Kokomi's CA gets a 12.2% hp buff (122% atk scaling) from Q, 6% hp buff from passive (60% atk), bringing 267% to 449% scaling (another nuke). These scalings are comparable to (higher than actually) Scaramouche's E buffed NA/CA. By contrast, Ayato's NA scalings are around 100%.
However, Kokomi cannot crit. Assuming a 'normal' character can reach 100% crit rate and 200% crit dmg (giving a crit multiplier of 3), this means her above mv is slashed by 3: $$z_b = x_b/3 = 163.1$$ Even if 200% crit dmg sounds too much, suppose a normal character can at least get 100% crit dmg (meaning 2 times the damage). Then given that Kokomi cannot crit, she is missing out on a 2x multiplier, so her actual dps is slashed by 2, becoming around 240 mv.
Above we did not account for possible vapes, but that would not fix her fundamental setback of not being able to crit. Even if she vapes all her NA (impossible due to ICD), the above would double to around 300 mv, which is still low. As such, Kokomi's potential as a burst dps is limited, while she is better suited as a support.
Her Q also buffs her E damage by 12.77% hp (around 127.7% atk scaling). Her E has 196.54% scaling. During her Q, this becomes 324.24% per tick, which again is high scaling. Her E can tick 7 times within its 12s duration, but say only 6 of those are buffed by her 10s Q. We ignore E reset. The E has a 20s cd. The mv is $$x_e = (196.54+6*324.24)*1.754/20 = 187.9$$
As her E has no icd, she can technically vape each tick. But the difference between Guoba and Kokomi is that Guoba can crit. The above mv is slashed by 3 if she cannot crit: $$z_e = x_e/3 = 62.6$$ This is like adding dust to dust.
If Kokomi's C6 allowed her to crit, then she would at least be decent. Otherwise, Kokomi is an example of a character with high scalings on paper, but who cannot leverage that due to inability to crit.
The 4p Ocean Clam set turns any healer into a physical sub-dps (the bubble pop does physical damage and is affected by physical res shred like superconduct, which enables Ocean Clam superconduct Qiqi to be viable in situations). The bubble pops every 3s, dealing damage based on 90% of the accumulated healing to all party members (so if multiple party members are healed, that will count multiple times to the total healing). There are limits though: only one bubble can appear every 3.5s, and a bubble can store a max of 30k hp healed (including 'overflow healing'). Moreover, only one bubble can exist at a time (so cannot stack multiple healers together).
These limits conveniently allow us to calculate a theoretical maximum to dps (ignoring physical RES shred). Suppose Kokomi can saturate the 30k limit each time (possible as she heals a ton). Then the dps is $$d_o = 30000/3.5 = 8571.4$$ To convert this to mv, divide by 4050 (see dps) to get $$z_o = d_o/4050 = 211.6\%$$ Now if we add Kokomi's main sources of damage, her mv is $$z = z_o + z_b \approx 399.2$$ Not only is this higher than Ayaka's dps, it is higher than Ganyu's. But that is not surprising given how weak Ganyu is.