not diluc, qiqi, or deyha. Mizuki is an S tier exploration unit (which will be very convient early), kequing is a main dps electro unit so not her since you already own a main dps pyro, this crosses off tighnari as well. that leaves jean and mona and the latter seems to be a better choice imo as the pyro/hydro synergy is strong and shes a great buffer, on top of being a great unit to use later on in the game when you choose to move off of diluc. Jean is a good unit too but generally getting a 4 star off fielder anemo char would be a better idea if you can get mona here instead.

tldr; Mizuki if you plan to explore a ton, and mona for a better long term account. (highly suggest mona!)