If you finished the game already, Satanael is the Persona you summon at the very end. On your second play through you will have the option to fuse this Persona through a group guillotine fusion (after May 18th).
Satanael absorbs curse, blocks bless and is resistant to all other damage. He has several powerful aoe spells that deal nuke/curse/gun/almighty damage, but the most useful ability is Victory Cry. This passive fully recovers the HP/SP on your protagonist after each fight. If you get firm stance on Satanael with Divine Pillar equipped, you’ll take only 12.5% damage from every attack. Because he starts at level 95 it’s not possible to max out his stats by abusing the sacrifice of other Personas to benefit him, but he is an excellent Persona to have in your roster.
If you’re doing it early on in your second play through you will need a lot of cash for this fusion. Somewhere around a million yen if you are low level. The fusion also requires maxed Lover’s, Justice, Judgment and Star confidants. The Personas you need to fuse are:
Arsene (1) – You start with this one.
Anzu (25) – Can be found in Futaba’s palace, the least expensive way to fuse one if you don’t have one yet is to fuse a Phoenix with a Regent.
Ishtar (85) – You need to max your Lover’s confidant to make this available. Cheapest way to fuse one is Raphael and Queen’s Necklace.
Michael (87) – needs rank 5 strength confidant. Fuse with Uriel, Raphael and Gabriel.
Satan (92) – Requires maxed Judgment confidant. Metatron with Ishtar can create him.
Lucifer (93) – Requires maxed Star confidant. Needs to fuse the following: Anubis (37) + Ananta (43) + Trumpeter (59) + Michael (87) + Metatron (89 maxed Justice confidant) + Satan (92 maxed Judgment)