The theories (don't think seriously about it, just for fun):
1.First Spirit will be revealed as an actually already revealed but unimportant character, either she was old or new character.
2.First Spirit is actually the one who told Kurumi to kill her because he/she can't kill herself for some reason.
3.He/she was unintentionally make first spacequake, but thank for "certain person" she end up make it and she really hate it and make him/her hate her-self (and lead to theory number 2)
4.First Spirit was not have any form from beginning. He/she was the being created from concetrated 10 Sephira Crystal that make him/her what she is. Haniel power to transform is the reason she probably have appearance of human.
5.Kurumi always strangely looks toward moon, where there's possibilities that actually she feels like someone she searchs for (AKA. First Spirit) are actually there. Maybe after Muku was sealed by Shido she was telling him that when in space she was saw a figure sitting on moon, looking toward earth.
6. In Progress
Well, that's all I can say. For remembering, this is just theories like other people make theories about it too so don't though it seriously. Happy day!