Thanks, but can you explain the fusion persona thing to me? At the moment I hardly ever use Personas in fights apart for healing. Their attacks don't seem to be any stronger than weapons and I need as much healing as i can get !
There is also only 10 slots, how do you know what ones to delete to make room?
Most foes have a weakness to a specific attack type - gun, melee or one of the various elemental powers of Persona. If you hit them with that an attack that matches their weakness, you can stun them, which opens up options to do multiple attacks or to negotiate. On the other hand, some are resistant or invulnerable to certain attacks. You can see these on your Personas under their name on the details screen - there is a long list of icons that show them - Nul means they take no damage from that attack, Str means they're resistant, Wk means they're vulnerable. When you unlock the ability to scan, you can see these on enemies too, until then it fills as you use different attacks. Life gets much easier when you can knock down 5 enemies with an single ice attack because they're all weak to ice
These relate to you as well, so if your equipped Persona is weak to fire, you're weak to fire as well. You can often use their resistances to your advantage.
When you fuse them, you combine 2 persona into a new one, but you get to retain certain powers from each - how many/which powers depends on the level of the new monster. As you go further through the game, you will find you can start to tailor Persona to specific powersets - so I had one that soaked up physical damage (can't recommend him enough - if I could remember the name/power it would be more useful...), one that had passive abilities for regen of HP/SP, one that had healing, one for status effects and recovery, one for physical/gun attacks and a few that had all the elemental powers between them.
Each of the NPCs that have a relationship level are assigned to a Persona type, so the better the relationship the better the fusion - you'll get bonus powers too. That gets more important around the mid game.
As you fuse them, the level of the new Persona goes up, which unlocks new powers - often new versions of the existing powers. Typically it's spells that hit multiple enemies, then a more powerful single attack, then a more powerful multiple, etc. The only downside is that the SP cost goes up. Your active Persona will also level as you fight - later on, there's a skill that let's the passive Persona get XP as well.
As for deleting them, you should be recycling them by fusing enough that it doesn't matter. After a while you'll delete them because you've already fused them with everything you can and they're just weak and pointless. Once they've been captured, they're in your Persona database (in the Velvet Room) and if you want one for fusing, you can buy them back out of it.