You get a specific set of SR party members as you play through the first couple chapters/regions; I listed them just a couple posts ago.
Otherwise, there is the Gacha system to sometimes draw new characters from(who also come with their respective weapon that your avatar character can use). You can also get Kamihime from some events - every event seems to have an SSR Eidolon and weapon, at the very least, that you can obtain. It's not particularly hard to get them, but getting enough copies of each to fully uncap/max out the Eidolon/Weapon is much harder. (Kamihime upgrade a different way - farming raids/special stages for materials.)
More power to ya. You can always class your avatar character as a healer if you find Nike isn't suiting your needs as well(she does have angel-wings if that's the theme you're after though). I would suggest planning that out ahead of time if you go that route though - the "EP" to unlock classes can be pretty tight early game.