Kallen カレン Showcase (Code Geass Collab コードギアスコラボ)
First impression isn't one of the best for this new free unit. Kallen seems to be a selfish attacker which kit is a bit awkward to use.
Her stack system at max stacks gives strong buffs (80% damage cap on ougi and 30%echo/30000 supplemental damage on autos), but you lose all stacks when ougi-ing.
I guess she can be useful for teams that focus almost entirely on auto (Evokers?), but for common usage you'll be ougi-ing every 2-3 turns making her stacks reset before they become something powerful. This is even worse when used together with Anila like it is in this video because of the charge bar speed buff.
She's definitely going to be a powerhouse for new players though because those players are unlikely to have great amount of DA/TA so she'll have more chances to stack up her power.
Her animations also very nice and it really shows the effort they put to make the character very appealing to fans of the series.