To add on, his refund is bonkers (it has the distinction of being one of the few, if not only, NP that has a lower NP gain on-hit on purpose).
HOWEVER, his default NP gain is utter garbage - arguably bottom 10% of all servants in the game. The only way he gains any real amount of NP is through ally boosting or Arts chains.
Kerry’s niche tends to be an NP looper in Arts teams that can fight in three types of battles well:
- Ones where a targetable taunt is important to bring in another servant or direct an ST NP to a particular target
- Teams focusing on NP drain for defense
- Need damage that bypasses defense buffs
Shiki tends to be more preferred because she’s all-around more consistent - better NP gain and a dodge so she has a fail-safe. Kerry works just as well most of the time, but tends to require more thoughtful play and has more extreme synergy/non-synergy.
So is it worth it? Maybe, if you don’t want to use Shiki for everything. They’re pretty interchangeable aside from specific fights where their niches can come into play. Kerry does have a better crit-niche since his invuln pierce applies to his normal cards too. The lack of a dodge hurts though, as well as a lack of battery on demand.
Last note, I wouldn’t recommend Osakabehime/EMIYA/Tamamo team. That’s only 9-ish Arts Cards including NP, and if you can’t consistently Arts-chain with EMIYA don’t expert more than 1 or 2 NP’s from him in a fight. I almost always run him on Tamamo-EMIYA-Waver team to maximize Arts Cards (11 including NP cards) and they can all NP drain for excellent NP lockdown.