1) After Always doesn't regret dieing, but he does regret abandoning his best friend.
2) Beyond Birthday regrets many things. He regrets not being able to stop After's death. He regrets the fear and worry he caused his brothers. He regrets being unable to protect Lawliet from himself. He regrets the deaths of the three people he killed and he regrets refusing all help for the madness that drove him to it. Most of all, he regrets never telling After how much he loved him.
3) Lawliet refuses to dwell on might-have-beens. Sure, he might wish he'd taken his family and run away before his title could drag them all under, he might wish he'd payed more attention to After's and Beyond's mental states, he might wish that he'd forced little Beyond to get help, that he could have saved his life, and Naomi's and Mail's and Mihael's, and he might wish he could have saved Light's sanity. Yes, he might, but he doesn't. There's no use wallowing in past mistakes, because the fact of the matter is that he's dead. And so are they.
4) Upon reflection, Light Yagami both does and does not regret being Kira. He's been a savior and a monster and though he feels shame at his actions, he also feels pride. Despite all the evil he wrought, good things happened too. Crime rates hit all-time lows, and, maybe more than anything, there was L. Really, if he wishes anything, he wishes he could have met L without destroying his family. He would have liked to work with him without he suspicion and fear. He would have like to see his sister grow up happy. He would have liked to be able to look his father in the eye.
5) Mail Jeevas doesn't really regret anything. He sometimes thinks maybe he could have helped A or B, he sometimes worries about Near all alone, he even sometimes withes he could have persuaded L to spend more time at Wammy's, for Mello's sake. Still, he's more or less content with how his life turned out. It wasn't the happiest of lives, or the longest, but it's the one he lived and he accepts that.
6) Mihail Keel is as emotional in death as he was in life. He has many regrets and is acutely aware of each of his failures, but even now, so many years later, his biggest sorrow is not giving his little sister the toy she'd wanted the day she and their mother died.
7) Nate River can never decide whether he could have changed things or not. He's watched the people close to him die and disappear all his life, starting with the mother he barely remembers and continuing on. After, Beyond – taken from him and then killed as well; a double blow –, Lawliet – always gone and back and gone and back and Nate hates him as much as he loves him, sometimes, and then he's dead too –, Mello and Matt disappear, most of the SPK dies – he doesn't care about them, not really, but they were there and then they weren't and it still matters –, Matt and Mello die and are followed by his – by L's – greatest adversary and then the Kira task force is disbanded and Nate's… not quite alone. He asks the remaining SPK if they would like to stay on with him and only Gevanni does, so that's two more gone. For each loss is a wish, and each failure, a sorrow. Nate regrets that he does not have the courage to persue his own dreams rather than one chosen for him. Nate regrets, wishes, fears, grieves, and hides behind layers of ice labled Near, N, L. And he regrets that most of all.