Broken Families - Luka and Adrien Mirroring
Way back when I compared how Luka and Kagami fit into the concept of a romantic rival, I made the claim that Luka was more Adrien's foil than rival, but didn't elaborate on it too much. However, the season four Jagged Stone reveal has made this foiling even more relevant, so I thought it was a good time to cover it. Spoilers for season four about halfway through this, there will be another notice right before I discuss it.
The foiling of Adrien and Luka happens through the theme of freedom vs responsibility, and this theme manifests in their family lives. Both Luka and Adrien have broken families with only one parent. This key similarity brings their differences into sharp focus. As Gabriel is cold and distant, Anarka is warm and approachable. The Agreste and Couffaine households just feel very different from each other. The Agreste Mansion is an isolated location where Adrien doesn't get to bring friends to simply hang out. Meanwhile the Couffaine Houseboat often serves as a meeting spot for Luka and Juleka's friend group, welcoming to all.
Adrien is the only son of a strict father. He can't have responsibility for anything in his life, because he isn't given any freedom to make choices to be responsible for. From his clothes to his hobbies, Gabriel controls everything about Adrien's life. Their home also exhibits this control by showing a highly designed, but otherwise empty home. There's no clutter allowed in the Agreste Household, nothing outside the facade of perfection. Adrien's schedule is also Nathalie's responsibility to maintain, with Adrien not being allowed pursuits outside of those things.
Meanwhile, Luka lives very freely, but also has to take responsibility for everything going on in his life. His mother is a free spirit, but also irresponsible enough to be very familiar with the local law enforcement. The houseboat the Couffaines live on shows this careless attitude in how Anarka refuses any cleanliness. Luka is free to do what he wants, and his mother merely supports him, but he is also clearly the responsible member of his family. He's the eldest child, with a part-time job, and he has been shown to act as an emotional support to Juleka. Luka has to take on responsibilities because his mother seemingly doesn't take them on much.
Adrien and Luka are both out of balance. Adrien has no freedom and no responsibilities. Adrien isn't free to make his own decisions about what to do, because his schedule is prepared for him, but he also doesn't have the responsibility of making the decisions of what his time goes to. Adrien is also confused as to who he exactly is as a person, because his life is so utterly monitored and controlled, he barely has any parts that are solely his.
Luka has all of the freedom, but also all of the responsibility, since his mother doesn't take responsibility for things. Luka has the freedom to make his own decisions, but also the responsibility to make them, because otherwise nothing would happen. Luka's identity is entirely his own creation, shaped by his environment but also by things he claims as his.
Now we come to the spoiler territory. With Jagged Stone being outed as Luka's missing father, Luka has to question a big aspect of his identity, his relationship to music. One of the first things we learned about Luka was that he was into Jagged's music, now he found out that Jagged put said music above his family. There might also be some concern over ending up like his father due to his own passion for music.
A similar identity-shattering reveal also looms in Adrien's future. Cat Noir is the one thing Adrien has claimed for himself. Discovering that his father is a villain and that Hawk Moth has been trying to restore his missing mother would shake Adrien's identity as explicitly a hero. To maintain the mirroring with Luka, the reveal of his missing parent's part in the conflict would be the most fitting trigger to force him to recontextualize what being a hero means to him.
This is how Luka and Adrien have parallel character arcs that foil each other.