General overview of gameplay mechanics of characters: this is like the path system of Star Rail or the role/job system of some other games like FF. As in other games, some characters can fulfill multiple roles.
Sometimes the best defense is offense, and the best offense is defense.
Buffers (harmony):
Debuffers (nihility)
Healer (abundance): Jean, Qiqi, Barbara, Kokomi, Yaoyao, Diona, Bennett, Noelle, Gorou C4 (not much), Morax C6 (not much)
Preservation
DoT should last for more than a few seconds (else classify as nuke)
Term sub-dps sometimes refers to off field dps or burst dps.
Talent used
Mode used
Uptime
Units with self sacrificing mechanism (cf Star Rail)
Continuous hp drain / modification (exclusive club of destruction characters, 'enlightened with modernity'): Xiao, Lyney, Wrio, Noctis, Focalors, Dehya, etc.
Besides hp drain (corrosion), continuous healing over time can also proc the special Fontaine effects/artifacts involving continuous hp changes. Continuous healers include (regeneration over time): Barbara (E), Charlotte (Q), Bennett (Q), baizhu (Q), Kuki (E), Jean (Q), Yaoyao (E), Focalors (E pneuma), Kokomi (E), Diona (Q), Sayu (Q), Gorou (C4).
For effects that want rapid hp changes, Yaoyao actually tends to work well (other healers may be slower).
Besides healing, eating food can activate certain effects involving hp changes (which gives another use for food).
Single instance of damage with high scaling (e.g. around 700% scaling or more)
Eula, Raiden, Mona, Hu Tao, Childe, Jean, Morax, Aloy, Lyney
Some bursts do DoT or are separated into multiple hits within a short time. The advantage of splitting a nuke into multiple parts is that it can avoid the 'crit fishing' problem (having a nuke miss the crit hit). Due to the law of large numbers (and central limit theorem), the more parts the nuke is split into, the more likely it will achieve the expected value of damage. On the other hand, splitting a nuke usually means no more wild damage per screenshot showcases (e.g. dealing 1-2 million damage in a single hit).
Diluc, Dehya, Alhaitham, Scaramouche, Wrio, Qing, Chongyun, Nilou, Ayaka, Noctis
Xiao, Raiden, Mona, Kazuha C6, Yae (through EM passive), Alhaitham (EM scaling), Nahida (EM scaling), Lyney, Noctis (HP scaling), Focalors (HP scaling)
Many of these characters get so much DMG Bonus that they benefit greatly from ATK buffs like Bennett (and crit buffs).
Itto, Noelle, Hu Tao
They buff their own ATK by converting HP or DEF. They benefit more from DMG Bonus buffs like Mona or Kazuha.
Dehya, Yelan, Nilou, Hu Tao, Morax, Kokomi, Ayato, Yaoyao, Baizhu, Noctis, Focalors
Tenacity set, HP weapons, HP sands
Itto, Yun Jin, Gorou, Albedo, Noelle
Husk set, DEF sands, DEF weapons
Cyno, Alhaitham (quadratic), Nahida (cubic), Yae Miko, Kuki, DMC, Kazuha, Tigh
Mona, Raiden
Emblem set, ER sands, ER weapons
Flat dmg self-buff (additive) is given by things like Yun Jin and Shenhe's buffs. We also call this dual scaling (see Dehya and Blade in Star Rail) as it can result in scaling on two different stats.
Childe, Raiden, Cyno
Shield on E: Noelle, Kirara, Morax, Layla, Diona, Thoma, Xinyan
Counter shields: Beidou, Candace, Yun Jin
Shield on Q: Beidou, Yanfei (C4 tankfei), baizhu
Game currently does not have: anemo shielder. And the hydro shield from Candace requires her to be on-field (not off-field), whereas the shields of other characters can remain when they are off-field. Also see the HMC's C4 shield.
Heal on E: Barbara, Yaoyao, Qiqi, baizhu, Kuki, Kokomi, Morax (C6), Focalors
Heal on Q: Bennett, Qiqi, Diona, baizhu, Jean, Mika, Kokomi, Yaoyao, Dori, Noelle (on-field), Gorou (C4), Sayu, Charlotte, XY
Self-healers: Lyney (E), Noctis (droplets), Fischl (C4), hu tao (Q), Wrio (CA)
Other: XQ (rainswords)
Single target healing: Bennett, Yaoyao (E), Kuki, Kokomi, Diona, baizhu (Q)
Teamwide healing: Charlotte, Jean (NA, Q), Barbara, baizhu (E), Yaoyao (Q), Qiqi (Q), Kokomi (Q)
Elements: there is currently no pyro E healer (could perhaps work like Kuki for burgeon), cryo E healer, anemo E healer (off-field), geo E healer.
Very rare ability reserved for a few characters: Jean, Beidou, XQ. These are applied to the team. Lyney became the 4th character with this ability (his Q applies 50% dmg reduction, but to self only).
Note Dehya's mitigation is a bit different. If Dehya's E gave like 80% dmg reduction, 100% poise resistance, 50% crit rate, 100% crit dmg, free 40% dmg bonus, 40% atk, 40% hp, while having 100% uptime, then wow...
It is the opinion of some that certain people may want to have a discussion over tea with the combat designers and devs of Genshin.