In Chapter 3, the students break off into groups of three, some of whom clearly play off one another in some manner.
Raphael, Felix and Caspar, in terms of brute force, are the strongest members of their respective houses and heavily invested in training. It doesn't take long at all for the three of them to request a sparring match with the other.
Ferdinand, Lorenz and Ashe are all members of noble families, though in this case the group is mostly that of a Foil, with Ferdinand and Lorenz being the foil to Ashe, who has reservations about his status and questions his own ability of whether or not he can lead while the former two are very confident in their own abilities.
Similar to Ferdinand, Lorenz and Ashe, the group consisting of Bernadetta, Marianne and Mercedes is also a great foil. Mercedes is like a mother hen towards the troubled Bernadetta and Marianne, both of whom suffer from a Dark and Troubled Past.
The group comprised of the House Leaders' Number Twos. Dedue and Hubert both possess Undying Loyalty towards their lords while Hilda is rather lazy, but is loyal to Claude. It gets taken a step further in regards to their relationships towards their lords also, with Hubert occasionally disobeying Edelgard's orders if he thinks it would be for her benefit, Dedue being unquestionably loyal and following every order down to the letter to Dimitri's exasperation, and Hilda willing to reel Claude back if she thinks he's going too far.
Lysithea, Lindhart and Annette are geniuses among their respective houses, so naturally it's expected they would be in the same group.
Fridge Horror
In the game proper, you get to see the consequences of what would happen if someone who did not have a Crest attempts to use a Hero's Relic. "Those Who Slither In The Dark" manage to one-up that by using poor ol' Kostas as a guinea pig by having Solon plunge a Crest Stone into his body, just to see what might happen if a Crest Stone is put into someone who doesn't have a Crest. And judging by the chapter's ending and Kostas' inhuman scream, followed by the Gory Discretion Shot, its clear the results aren't pretty.
But for those who have seen the Crimson Flower route, where willing Kingdom Soldiers and Dedue (against Dimitri's wishes and behind his back) voluntarily become monsters to hold off your army via use of Crest Stones, they might have a closer guess as to the results.
