Fafnir is probably the best overall choice from the pre-reg. Its summon effect is attack/defense (large) boost that lasts 2 turns, and it does not overlap with buffs from your princesses or spirits.
In my opinion, you have won the lottery there.
In this game, a full party consists of a Spirit, 4 Main Princesses, 2 Sub Princesses, and 6 Eidolons. Every round, you will be able to use abilities from your Spirit/Princesses. The main difference between a Princess and Eidolon is that even when abilities are on cooldown, princesses can still attack and deal damage, while Eidolons can only be summoned when their cooldowns are off.
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Chronos is an interesting one, its second ability is nothing special. But its first and third abilities are its bread and butter. Its third ability can boost the damage of its first ability. But the best part is that the third ability can be stacked, so if stacked enough in a long boss fight, in can actually deal some significant damage for a mere SR.
If you plan to main wind element, that can be considered a keeper.
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Not allowed to post links, but you should be able to find it.
神姫プロジェクト攻略まとめwiki