Of the Keterburg gang Jade and Peony are the only ones who stayed together. This shows how deep their friendship is. It is rare for Jade to stay or respect anyone fully because of the sins of his past but even his sister would say that Peony is the reason why Jade changed and is now able to live a fairly normal life. Peony doesn't take credit for this accomplishment. In fact, he tries not to talk about it because the subject of fomicry still weighs heavy on Jade's mind. Peony also doesn't blame Jade or even fear him for what he's done. Seriously, the guy killed monsters for fun, invented fomicry, and cast fonon inscriptions on his eyes, when he was nine. Needless to say Jade was odd and a bit twisted,. twisted enough to drive his own sister away. Peony stayed by him though. (considering you can't count Dist because he is just as twisted if not more so). Also when Jade realized the value of human life Peony still stayed with him thus showing that he wasn't staying as a mentor but just as a friend.

Later Jade was adopted into a military family, one that serves Peony. Now, some believe that it was simply because he had scared the people of Keterburg to a point where they had to take Jade away from there but my opinion was it was either Jade's wish so that he could stay with Peony so that he didn't start fomicry again (Geniuses like Jade get bored very quickly; you know) or it could have been that Peony was lonely and wanted Jade to be near by or that he wanted to take Jade with him because Nephry still feared him a bit. (Peony loved Nephry). All of the above could have had a role in it though.

Overall, the situation only allowed Jade and Peony to get closer and now Peony can match wits with Jade in an oblivious way when he responds to Jade's snark. It's probably the reason why the one Jade respects most is Peony.