Raine is an essential member of the party, especially on a first playthrough. Although she can only use healing magic early on, once she learns Photon she'll
have some powerful attack magic as well. While Kratos and Zelos do have First Aid, I suggest using them mainly as fighters and letting Raine do the healing.
Raine is probably better left using Long-Range magic, and staying towards the back.
Raine is best raised as a T-Type. S-Type spells are used to heal only one character at a time, while T-Type spells will heal the whole party.
She'll learn Revitalize as a T-Type, which is the best and most useful healing spell in the game. I do prefer Holy Lance to Ray, though, so on future playthroughs
I suggest shifting her to an S-Type after learning all the T-Type spells, and forgetting Ray
and learning Holy Lance. There is a way to get both S- and T-Type spells by means of a glitch,
but I haven't gotten around to trying that for myself yet.