Yeah, but no RNG.
If everything was random, then you could get attack buffs from spirits with resistance abilities, which is not going to happen.
Here me out. It already is random to you, because nobody knows anything, and it's not like you all had a massive strategy behind your picks. It was mostly "hey let's pick Fire Emblem", so technically it will be random if you select your favorite characters/choose from your favorite series. Keep in mind that most spirits are usually 2-slotted, second most spirits are 3-slotted, third most are 1-slotted, and the least amount of spirits are 0-slotted. That is also the order of the worst to best abilities; less slots means better ability, at the cost of Support Spirits. 3-slots also have better abilities than 2-slots only because of how fragile they are in match ups. No matter what happens though, I can confirm everything is fair and everything has already been verified as such by yours truly, the spectators. So yeah, while I would love to see variety, everything still is fair even if many people have picked similar spirits.
But just in case, here is a quick rundown how all the abilities work:
Aggressive players should pick Attack Spirits. Protective players should pick Defend Spirits. Controlling players should pick Grab Spirits. Analytic players should pick Neutral Spirits.
TL;DR
If you want randomness, pick your favorite spirits. If you want to be in charge of your own game, re-read the above statement. I will not randomize the abilities on a spirit-to-spirit basis, because the abilities that I have wouldn't be logical.
I'm giving out some public Support Spirit abilities as examples.
If a Support Spirit gives any type of battering item and takes up one slot, you will deal +1 damage
If a Support Spirit gives any type of shooting item and takes up one slot, you can deal 1 damage to another player of your choice
If a Support Spirit gives any type of throwing item and takes up one slot, you will deal 1 damage on the subsequent phase
(These are the most basic spirits my Support roster, and are only 3/52 that you will find. They are also debateably the worst, but that's beside the point. I wouldn't reveal the "best" Support spirit)
It wouldn't make sense for a spirit that gives you a Killing Edge to heal you, which is why all of the Support Abilities are reflective of the actual ability on the spirit. I can promise you that that will not change. Support Spirits that consume more than 1 slots will logically be more powerful/valuable, or an upgrade of a similar 1-slot Support Spirit.