Let me cook:
1. He saves her
2. She remains in the same world as him
3. She probably falls in love with him
4. He is still fused with this world's Saika means he can change back to Saika
5. Saika x Saika :questionblob:
Really, you leave this manga for 5 fucking months, then when someone posts a new chapter, you suddenly have the motivation to start translating this series again, fuck off with this bullshit.