Trumpkin and Nikabrik are both dwarves and neither of them really buy into the whole Aslan thing. Kind of odd that they're skeptical of a talking lion while sharing a room with a talking badger, but we digress. These two serve as each other's foil not because they are skeptical, but in how their skepticism leads them to act.
Nikabrik's skepticism leads him to question Caspian's decisions. The black dwarf abandons duty and honor, refusing to follow orders (7.57) and attempting to resurrect the White Witch, source of all evil and an all-around uncool lady (12.30).
Although Trumpkin is skeptical, he never lets his skepticism get in the way of his duties. As he tells Caspian, "'I know the difference between giving advice and taking orders. You've had my advice, and now it's the time for orders'" (7.60). Trumpkin remains loyal despite being unsure that the loyalty will pay off.
If you thought Miraz was a slice of evil cake, you should check out Lord Glozelle and Sopespian. These guys are power hungry and will even turn traitor if it suits their needs. As Glozelle tells Sopespian, "'[…] Miraz is no very great captain. And after that, we should be both victorious and kingless'" (13.34). In other words, why give the king cred when you can have all the cred yourself? They also act as foils for Peter and Edmund.
Unlike Glozelle and Sopespian, Peter and Edmund aren't fighting for their personal gain. As Peter tells Caspian, "'I haven't come to take your place, you know, but to put you into it'" (13.77). He'll fight for Caspian's kingdom because it's the right thing to do.
The differences between Peter/Edmund and Lord Glozelle/Sopespian is also super important to understanding the theme of "Principles" or chivalry. So pay super-extra close attention to these guys.
Both girls are main characters in their own right, but Susan and Lucy are also foils for each other. Susan is beginning to grow up while Lucy is still very much a child. When Lucy sees Aslan before the others, Susan asks her where she thinks she saw him. To which, Lucy replies: "'Don't talk like a grown-up. […] I didn't think I saw him. I saw him" (9.63).
Susan's more grown-up qualities are definitely not as admired as Lucy's childish ones. Whereas Lucy sees the proper way to go, Susan's adult thoughts help lead the group astray. At Cair Paravel, Lucy's desire to explore counters Susan's desire not to, leading to the discovery of the treasure chamber. Finally, Susan is the last Pevensie to see Aslan.
In relation to the theme of "Coming of Age," Susan's character suggests the novel's hesitation of growing up whereas Lucy shows the value of not.