The Story behind Devon's relationships.

Truth be told, Devon and his friends weren't always caught in the semi-love triangle they're in during KiSy.
This all began a couple of years prior to the events of KiSy, with Yuuki entering the school as a new student. Because of her position as an ambassador's daughter, a lot of people were scared of Yuuki due to rumors stating that her father made people disappear if they dare make her mad. (Truth is that Mr. Watanabe could not give a single fuck about what happens to his daughter). These rumors were eventually dispelled, however, by Devon.
Unlike others, he was bold enough to approach the girl and offer his hand in friendship. Seeing as he was her first friend aside from Fuyuki, Yuuki quickly clung to Devon and grew close to him. To Sean's charigne, they grew too close. During this time, Devon began to spend less time with Sean and more time with Yuuki. This got to Sean, who realized that perhaps he liked Devon a lot more than a supposed 'brother' should love.
As luck would have it, the day Devon decided he wanted to make things official with Yuuki, Sean decided to act on his feelings. So, before Devon could leave to talk to the girl, Sean whisked him away on a date. It was in this date that Devon also came to realize that he loved Sean just as much as the man loved him.
This became quite the conundrum, as Devon could not bring himself to choose between them. So, moronically enough, he decided to spend time with the two of them at the same time, hoping that one of his interests would graciously bow out for the other. That did not happen. Instead, the two dug their heels in deep and clung even more to him, as neither would relinquish him.
This stalemate lasts for a year, and reaches a conclusion in KiSy. Marie knows of the issues that plague the two, and so she approaches them separately and offers them a deal: work for her and ensure their escape with Devon. Naturally, the two of them take the offer, neither one knowing that Marie was going to pit them against each other...

Conclusions:
In the True Friendship End, Devon comes to realize just how deep the wounds he's inflicted on his friends, and thus decides to bridge the differences between Sean and Yuuki, mostly by coming between them during their final fight and taking their blows. After calming them down, he promises to settle things once and for all after escaping.

After the credits, there will be two different scenes depending on your relationship values. If either Sean or Yuuki had a higher value,then the end will have Devon talking to the two of them in his home, telling them that he doesn't forgive himself for driving them to nearly kill each other and thus decided to end things. He offers them an ultimatum: they can chose to move on from him and find someone more deserving of him or wait for him to forgive himself, possibly risking him never returning to them. The one with the lower value will kindly bow out of the race (With Yuuki tearfully thanking Devon for being her first friend, or Sean tearfully returning to his role as Devon's brother) and leave. Shortly after that, Devon will walk out and sit in his backyard. Here, the higher value one will approach Devon and tell him that they haven't given up on him, and will wait however long it takes for Devon to forgive himself. They seal this promise with a kiss, and a hope to share even more when the day comes.

Should the values be similar or close to similar, then the final scene will occur at an airport. Devon is ready to go to college, and Sean and Yuuki have decided to accompany him to the gates. Here, Devon will say a similar speech as he did in the former scenario, in that he doesn't forgive himself for what he drove them to do. But he adds that he considered having the two of them. After all, it's not like he loved any other woman or any other man. But after seeing just how far their jealousy would drive them, Devon decided against the idea. He tells them the same thing he said before, in that they can decide to move on from him, or wait for him to forgive himself. Before either could answer him, the plane he will board is given its last call, and thus Devon leaves them to decide for themselves.
If their values were between low and middle, the two decide that, although they won't pursue him romantically anymore, Sean and Yuuki will still wait for their friend to return.

If you somehow got both of them to max value, then the two will agree that neither of them will give up on being with him. And that if Devon is willing to learn to forgive himself, then they too can learn to trust each other more and staunch their jealousy so that Sean and Yuuki can both have him.