Is jalter or raikou stronger

In terms of game, they have different roles. You can't compare a ST damager to an AoE farmer. In terms of lore, I don't have the answer.

I feel like J Alter is self sufficient.
She has invilsibility, she can buff herself

Raiku can buff itself too but uniqe point of Raiku is "Increases own critical star absorption for 3 turns." so he can make strong critical hits.
But for that you need critical points so you will need someone to provide it.

I rolled for Raiku for critical star absorption skill and I will try to roll Merlin (and Zhuge Liang too) for support.
But I think I would be more than happy with Jane Alter and probbaly use her on most cases instead of Raiku.

As crit monster? In her niche of killing Demonic, Sky and Earth servants, Raikou is obviously stronger. Outside, Jalter, of course.