Vivienne

Vivi is a fairy caster whose useage is quite versatile, as she can act as both main DPS or sub DPS + support. Her biggest asset is the teamwide speed buff subskill Westerly, but she also applies an Atk buff to the furthest ally from her with Snow Eater. Even at low ascension, the speed buff alone can often give your team the boost it needs to win a fight that would otherwise be much more difficult. She also provides invaluable support for slower units such as Honglan. Her capabilities as a main DPS are in high regard, as she permanently increases her own crit rate by 5% (up to 5 times) every time she uses her main skill. She is also innately quite fast thanks to her mana regen passive and artifact, which means she ramps up quickly and scales well with crit damage. The only thing holding her back is the limited range/area of her skills. In team play, she needs good positioning and/or ally support to maximise her damage by getting the enemies in a position to be hit by her straight line main attack, and mid-sized AoE ult. Her team buffs and relatively high personal damage make her very useful for raids and other boss fights, although she will often need higher investment to stay alive because she is quite squishy. She's also a good unit for PvP for all the same reasons - her kit is well-rounded and very useful for her team.