Maybe I am discussing too primitive, but I do not agree with anyone who thinks that there is a possibility of final another than Zenkichi x Medaka.
It certainly exists, but it -> 0 (in term of mathematic)
There are many explicit and implicit evidence that:
- Medaka is the main love interest Zenkichi
- Zenkichi is the main love interest Medaka
Re-read the chapter 145 - Zenkichi says absolutely clear -
http://www.mangareader.net/medaka-box/145/13