Haru's galaxy brain plan to ensuring she gets to keep Tenka to herself:
1) Get isekai'd
2) create a situation in the other world for which the only solution is to use a magic item she made
3) ensure the magic item only works for Tenka, and ALSO requires he stay pure until she can "retrieve" him
4) convince a goddess to go along with the plan and both isekai Tenka and structure her religion around steps 2 and 3
5) Make sure Tenka solves the problem she created
6) schlick furiously until Steps 3-5 happen