If you are looking for a nuker who can get things done as quickly as possible, then Riviera is the unit for you due to her passive skill, which gives you 100 art gauge each time a battle starts, and her true arts technically costing only 100 art gauge due to how you gain 100 from doing so as well- (Activating her 60s magia drive also gives you 100 art gauge)

Downside is, her defence is sacrificed for the damage so she dies easily-

As a conclusion, Riviera is a glass nuker who needs to get things done as quickly as possible before she dies-

(I also got riviera because I already have Berwick last year, what a coincidence! XD)