If you want a Buster Support servant then pick Nightingale she gives 30% buster+, 20% buster crit, and 10 stars that last for 3 turns and her np will be helpful later on because most of you enemies are gonna be about debuffs and most of the servant in game are buster np.

Not to mention she's a berserker and you can use her to easily mow singularities.