It doesn't matter how many lines it is, it's written crystal clear. That is what writers call foreshadowing. There's no other way to interpret that than for it to be Windy. It's a very subtle way to put it out there without saying it in blunt words, foreshadowing.
1-Luc uses the same power as Windy, the only person capable of creating an army of illusions.
2-Yuber has always served Windy
3-He mentions a mentor teaching him a power and Leknaat never created monsters.
4-Windy is the evil half of the Gate Rune so why wouldn't he change mentors when he changed?
5-Luc goes inside and betrays Harmonia; Windy has a passion to destroy Harmonia.
All of this evidence suggests by WHAT THEY SAY in the actual game that Windy is in fact the one who made Luc change the way he sees.
Play my game to get a better understanding. it's available on the game's facebook page.
-link deleted, putting this up multiple times is akin to advertising - Op
Windy was never Luc's mentor. He was raised and trained by Leknaat. As for your bullet points...
1. Windy didn't have the power to create illusions, That's Sarah's unique power. The power to summon monster with the Gate Rune/Blue Gate Rune is something Windy and Leknaat could both do.
2. Yuber didn't ALWAYS serve Windy. It seems that after the events of S1, Yuber began serving the Silverberg family because in S2, his was recruited to the Highland army by Leon Silverberg, and then Albert Silverberg in S3. He has no connection to Luc at all.
3. We never saw Leknaat conjure up monsters, but as a bearer of part of the Gate Rune, it's something that she could have easily done, and there's no reason to think that she didn't show Luc that power in the 30 years that he was her apprentice.
4. Windy's half of the rune wasn't "evil". Windy herself wasn't really evil either. She was seeking power to exact revenge on Harmonia for the destruction of her village. Ironically, in that quest for power, she was guilty of committing the same crimes as Harmonia. If I recall correctly, Leknaat's abduction of Luc from Harmonia was motivated by revenge against Harmonia as well. I wouldn't call Luc "evil" either. He simply didn't want to be a slave to fate and chose to fight his destiny.
5. Luc's betrayal of Harmonia wasn't personally motivated, and it wasn't his ultimate goal. He only used his position in Harmonia to to draw out the hidden elemental True Runes to complete his plan to destroy his own rune.