Does Thad have any thoughts or feelings about his time as Bart? He made what he believed was real connections with people, including a home with Max and Helen and friendships with the Young Justice team. Even Dox. What did he feel while it was happening, directly afterwards, and now years later?
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As my version of Thad is now, he finds the time when he was impersonating Bart to be bittersweet, there were good moments and bad moments, though sometimes it is just completely bitter for him.
The part of him that finds some sweetness in that time is the part that liked being a hero. He liked the praise from the people he helped, he liked that he was sorta good at it, and maybe if he stuck with it long enough he would have come to associate those feelings with liking that he could help people and make a difference in their lives.
While the rest of the experience is bitter because he doesn't see the connections he formed as genuine (and they're honestly not).
The way he views the connections he made with Helen and Bart's Alabama friends is that they weren't exactly real. As a kid he wanted them to be real, but as an adult he understands that these people didn't actually love or like him as himself. They just thought he was Bart. From his perspective now as an adult, he just looks at that time logically and clinically, and understands that he craved friendship and familial relationships as a child that he didn't receive from his own family and in being raised in the 30th Century. He now knows that their feelings weren't towards him as Thad, but towards Bart.
Because they didn't know him as himself. Sure parts of his true identity would slip out, but for all intents and purposes he was trying to be Bart and they saw and treated him as Bart.
Of everyone he has the most positive feelings towards Max albeit in a retrospective sense. Max may not like him, or what he's done, or even like the person he is now, but Thad likes and appreciates Max because he tried. He tried to see him and help him.
And with regard to Dox, I'd say he was ambivalent/neutral towards him during the time since he pretty much was just bribing the pup with treats the entire time so he wouldn't blow his cover. Now he would probably actually like the dog/try to get to know him a bit.