I am talking about the character that seemed to be just a guest star at the start (or when they started) but developed into a good character during the whole series or end.
This is a hard one. Please explain why you selected that character.
How was Sisko "just a guest star"? He was the captain on a Star Trek show! Under these terms, he was never going to qualify for this poll!
I voted Other for Miles Edward O'Brien and justify this according to the terms above because when he started on TNG, he was just a guest/extra and his rise to prominence on DS9 was never set in stone: he could have been left on the sidelines as just the plucky, reliable, everyman who occassionally lends depth to a scene in ops.
He's also got fan credibility in the sense that it was the viewers who gave O'Brien/Meaney his break on Star Trek through the write-in campaign on TNG.
Someone (perhaps several someone's) once said that Worf
is Star Trek. I think not. I think he's second place to O'Brien and as evidence, I could cite numerous episodes but will limit myself to O'Brien's confession to Bashir that he killed his cell mate in the Episode
Hard Time (which I just happen to have watched a few minutes ago

). I quote it below from Memory Alpha and, if you think about it, the ideas contained within this short speech and Bashir's reply are gold-plated examples of the Trek philosophy as it has been since day one, episode one:
"
When we were growing up, they used to tell us... humanity had evolved, that mankind had outgrown hate and rage. But when it came down to it, when I had the chance to show, that no matter what anybody did to me, that I was still an evolved human being... I failed. I repaid kindness with blood. I was no better than an animal."
"
No. No, no, no. An animal would've killed Ee'Char and never had a second thought, never shed a tear... But not you. You hate yourself.You hate yourself so much you think you deserve to die. The Argrathi did everything they could to strip you of your humanity and in the end, for one brief moment they succeeded. But you can't let that brief moment define your entire life. If you do, if you pull that trigger.. then the Argrathi will have won. They will have destroyed a good man. You cannot let that happen, my friend."
-
Julian listens and sets things right with
O'Brien