Mika has the epithet 晴霜的标绘 qingshuang de biaohui. First, this can refer to a certain rebellion in Taiwan that was quelled during the Qing dynasty. Given his va, the epithet could also refer to how Deng Xiaoping executed Zhu Guohua, the grandson of Zhu De. The fact that Mika is a physical buffer that can support Eula gives further credence on this line of interpretation.
The only thing that does not make sense is why Mika was released on Ayaka's banner in 3.5. Mika does not synergize much with Ayaka.
His E Starfrost Swirl is one of the most unique abilities in the game. First, Starfrost Swirl is a crossbow shot. Moreover, it grants soulwind which gives 22% ATK spd bonus (lv10), which is actually better than the spd buffs from C1 Scaramouche and C1 Cyno. At lv13 it becomes 25%. The soulwind buff lasts 12s, and the E has a 15s cd. This universal atk spd buff really makes one question why certain constellations exist.
With his passive, is E can also give detector stacks if it hits more than one enemy. It will give one stack for each additional enemy hit. Each stack will give 10% physical dmg bonus, up to three stacks (four at C6). So if there is only one enemy, then 0 stacks will be gained. If the E hits four enemies, 3 stacks can be gained. This changes at C2, where hitting one enemy with E can give one stack.
His Q has 70 energy cost and heals the party. Doing NA will also heal the active character (eagleplume healing) once per 2.5s, lasting 15s. With another passive, if he has his Q and E soulwind both activated, and the active character crits, then they gain one detector stack.
It is unclear why Mika has an aoe limit on his E. Although it is true most characters can do physical damage, the detector buff is only 30-40% physical dmg bonus at most. It is not like this would make Mika too broken, considering Mona's Q can already give 60% universal dmg bonus.
In the menu, Mika kicks his polearm, joining the polearm-kicking club with Xiao and Cyno.
C1 decreases healing icd on Q. C2 gives extra detector stack. C4 gives 15 energy to Mika. C6 allows up to 4 detector stacks and gives 60% physical crit dmg to characters affected by soulwind.
Mika needs C6 to be used as a significant physical buffer as 30-40% physical dmg bonus is not enough to justify his paycheck. Without C6, Mika gives healing and the universal ATK speed buff.
Mika can support any physical dps, including: Eula, Rosaria, Razor.
For his universal atk spd buff, Mika can technically support any character in the game. These include: Yoimiya, Cyno, Diluc, Scaramouche, Alhaitham, Childe. Itto could also work, but Itto already buffs his own atk spd. Xiao could work for NA/CA, but atk spd does not appear to buff plunging or E speed.