What is the relationship between Odin, Seifer and the Kanji that appears?
The kanji that appears when Odin uses his special attack is 斬鉄剣 (Zantetsuken).

However, when Seifer kills Odin, it shows 斬鉄剣返し (Zantetsuken-gaeshi) which literally means "Zantetsuken Reversal". -(k/g)aeshi is a common suffix to mean "reversal/counter", especially in martial arts like Judo (e.g. Tsubame-gaeshi).