Don't worry about reworking synopses into character histories; we do it all the time. Plagiarism's only an issue if you're taking writing from another site.
Thank you again for all the plots you've been writing. I do ask that you keep a few things in mind, however:
- Plots are written in the present tense, while character/kaiju/vehicle/organization/etc. histories are in the past tense.
- Give your synopses a once-over before publishing them; there were quite a few typos in the last one. I will admit that I constantly forget to do this myself!