Iris and Pan.

It is very easy to have then both, it will cost you just a little bit of time to farm 60 essences in the cloud city, essence of water for Iris by killing little fairies flying around, essence of wind for Pan by killing a pair of ... horn flying around. The orb of Iris is a reward for login, so at the start just farm the essence for Pan. You will need a team of 3 sylph for the sylph arena.
Ultimately in the first day or so you can farm about 20 of them if you are dedicated. You can feed your main one with the newly created (for exp) or just sacrifice them (for sepulcrum). That is if nothing change in a week or so.

Try them both as your main one and decide for yourself which one you like, i saw some knight with Pan and other with Iris.
You attribute skills point gained by leveling them to strength, armor, intel or endurance. You can enchant them using sepulcrum obtained by sacrificing a sylph, by reward of the sylph arena or quest rewards. You level your main sylph (the one equipped) when your own toon is gaining exp (with MP, SP, bounty quest, quest, necro and cata) as he is gaining the same amount of exp.
Iris and Pan are rather weak, not sure at higher level but pretty sure some more powerful ones will be implemented later

As far as skills are concerned, the ones your pet gives you when equipped but not awaken (you are not transformed) are different from the skills they have when awaken.
Again, keep in mind that this is what we see/read but it is in test, so all could change or being incorrect

All are level 1

Pan not awaken:

Wind protection (passive) : increase player HP by 5%
Whistling blow : 300% +100 physical wind damage to a front row single target. Damage ignore defense and cannot be dodged
Cyclone : 280% + 125 physical wind damage to back row. Damage ignore defense and cannot be dodged

Iris not awaken:

Water restoration (passive) : Increase player MDEF by 3%
Borealis danse : 300% + 100 magic water damage to single front row. Damage ignore defense and cannot be dodged
Icicles storm : 190% + 80 water damage to all ennemies. Damage ignore defense and cannot be dodged

Pan awaken (when you transform)

Wind grace : cool down 30 sec. 145% + 200 physical wind damage to 2-4 ennemis . Ennemy defense reduced 50 pts for 3 rounds.
Delphic Apocalyptic hurricane : cool down 45 sec. 314 % + 350 physical damage to all ennemies
Gust strike : cooldown 1 sec. 136% + 180 physical damage to a single front row target

Iris Awaken (when you transform)

Warm spring : Heal 85% + 300 magic damage to teammate with lower HP ... i know it says it damage team mate but heh ... that's what i read lol
Deluge strike : Deal 165 % +180 magic water damage to a single target
Delphic - Acheron wave : 280 % + 350 magic damage to all ennemies