Ultimate Personas can be fused after you've obtained their respective key items. If you're going for a complete compendium in Persona 3 Reload, you'll want to spend as much free time as possible on Social Links. Since you'll be juggling more than 20 Social Links, you may not unlock these Personas until late-game. Among these are Atavaka, the ultimate form of the Strength Arcana, and a worthy candidate if you need a Persona that uses Physical Skills.