Persona 3 - What is a Persona/What is a good Persona?
Persona 3 is built around the concept of Personas if you couldn't already guess. If you have ever played a Final Fantasy game, Personas are kind of like summons/eidolons which you equip to your character and they let your character use their magic. There are many different types of Personas and many different types of Persona abilities in this game, I won't cover them all here but I will give you a good run down of what is out there.
Personas gain experience as well as your main character as he defeats monsters in combat. You can also earn experience/levels with a Persona when creating it if you have a raised Social Link. A Social Link is a link you establish with a character in the game when you become their friend. As your friendship grows so will your Social Link. If you would like more information about Social Links check out my Social Link guide.
In Persona 3 there is a total of 22 different Social Links that you can establish and raise. Here is a list of all 22! If you'd like more information on each Social Link, such as how to start it and information about raising it click on the name below!
What a mouthful, huh?! You may recognize where Persona 3 got the different Persona types from; it's mentioned sometime later in the game during a lecture but each of the different Arcanas all correspond to what we know as Tarot Cards. This will be an answer in one of the classes during school!
Alright so as you could probably already guess each Persona category has its own strengths and weaknesses. I won't go into massive detail for each because I feel that to do so would be a waste of both of our times. Throughout the game you'll be cycling through Personas based on what you have available to you and what Personas you've made in The Velvet Room depending on what mobs you are facing. You'll learn most of this stuff yourself and learning it yourself is pretty much part of the game.
What makes a good Persona?
This is a question a lot of new players probably ask, and I feel I can do a decent job pointing you in the right direction. Essentially the best Personas you can get are the ones with the most different type of attacks (the more elemental attributes you can attack with the better). Also, strong Personas will have AoE spells which will make it even easier for ditching large packs of Shadows.
When I get a Persona or am looking for a good one the first thing I look for is how many different attack types it has followed by if it has any AoE attacks. Then usually I look for Auto-abilities which are abilities that automatically activate at the start of each battle. These are great as well, as you could probably guess.
The main thing I try to avoid is Persona's with an alarming amount of Physical attacks. Aside from the attack saying it is a physical attack you can easily tell it is because it consumes your HP not your SP. I try to avoid these types of Personas because you already have physical attacks with your weapons and generally mobs aren't weak to them... If they are, well, your weapons do the job of knocking them down just fine and they don't consume health to use.
About your Allies' Personas
It's important to keep in mind that your allies and their Personas don't work like your own. They can't change their Persona like you can and they're stuck with the same one the whole game. Some of your allies' Personas will evolve during the game but they can't change types all willy nilly like you can. Because of this it's really important for you to learn and memorize what your allies are capable of.
Here's a brief run down of a few of your allies throughout the game. Not every ally you get is listed here as I didn't want to spoil anything major.
Junpei primarily uses Agi (Fire) and Melee attacks even when he should be using Fire cause he's a stupid. The only redeeming quality of this character is he's voiced by Vic Mignogna. Despite me really liking Ed from Fullmetal Alchemist voicing Junpei I normally didn't bring him along with me to Tartarus because I felt he was the worst character of the whole team.
Akihiko - Polydeuces (The Emperor) ; Caesar (The Star)
Akihiko is one of your primary physical damage dealers. His Persona has Zio (Lightning), Sonic Punch (Strike), Dia (Healing), as well as a multitude of debuffs Tarunda, Rakunda and Sukunda. There are some instances where his debuffs will be a big help if you can't do them yourself.
When I encounter a boss I usually set Akihiko to "Knock Down" this way he doesn't waste a turn helping or healing someone. He does more damage than Junpei and Yukari and is on par with or slightly above Mitsuru.
Mitsuru doesn't start off as playable but once you get Fuuka she'll be available as a battle party member. Much like Akihiko she will be one of your primary damage dealers and is all around more useful than Yukari and Junpei. Mitsuru uses Bufu (Ice),
Yukari has Garu (Wind) and Dia (Healing) primarily.