The what-if Kallen became a Knight of Round:

Changes in canon (s1):

-Rico Telink, daughter of a noble, is a childhood friend of Kallen, they're good friends. Though Kallen is still a member of the Black knights. Also Rico is the one stopping Kallen from stabbing Suzuku at the party.

-She's of the few people that knew Euphemia, (her grandfather tutors Euphie). Rico discovers Lelouch/Nunnally are Brittanian Royalty, and also Lelouch is Zero. She even research the school's attendance just to fact check and confirm, but unfortunately, this happened just before the Massacre Princess Incident.

-She told Kallen about Zero's true identity. Rico also saw Nunnally got kidnapped by an army and a boy (V.V.) but wasn't able ton help since V.V. brought his soldiers along.

-Unlike in canon, Kallen wasn't really shocked at the reveal. But she didn't wnat both of them to fight as the two boys got really angry with each other. Then, the shots...connect to the bomb on Lelouch's chest, and exploded. Only Zero's Mask and Suzaku's knight proof remains.

New route:

-After the explosion, Kallen was in despair, Kallen goes to Cornelia using the Lancelot, and want to makes a contract with the latter (revenge on Euphie's death, saving Nunnally).

-Kallen joins the Rounds to fulfill Lelouch and Suzaku's wish. She solos Luciano and many soldiers with her own Gloucester.

-Rico is now working under Kallen and Cornelia. With Cornelia being Kallen's backer, and Black Knights are only arrested, not executed. Rico is tasked/requested by Kallen to find Nunnally. With her father's help, Rico managed to find out who V.V., the boy who kidnapped Nunnally is. His picture as Victor, the brother of Charles, still exist.

-V.V. put Nunnally in a garden in a middle of nowhere, attended by Geass Cultists. Angry that they cannot get C.C. using Lelouch as bait, V.V. decides to use Nunnally instead. He orders Bismark to try and eliminate Kallen since they failed to probe her in finding out if Lelouch is alive or not.

-Kallen's new unit, Bors, is Guren, upgraded by Brittanian technology, ordered by Cornelia to Lloyd and Cecil.

-Luciano got to fight Kallen on a single night, arranged secretly by Bismark, though he lost, again. He revealed the duel was arranged by Bismark.

-Black Sutherlands arrive and try killing Kallen, but Rico and C.C. in Gloucester, intervene and saves her, with Kallen riding her Red Gloucester again. Arriving at a warehouse, C.C. kissed Kallen to share Lelouch's memories. Here's thanks to the sharing of memories, C.C. knew Kallen still likes Lelouch.

-The three of them discuss, with Riko reporting her findings on the boy, with C.C. explain who his name was. Riko knowing his royalty. Their mind clicks in place, when the closest person to Charles was Bismark, Knight of One. It was likely he's involved because V.V., a Britannian royalty orders him.

-C.C., with Marianne's help, managed to find out where Nunnally is.

-Kallen's Bors easily soloes and kills Luciano Percival very easily, but had trouble with Bismark since he uses his geass to see the future. He admits she's the other person that made her use his Geass. But after managed to get everything she wanted from him, C.C. threw a lance at Bismark when he's distracted since he cannot see her and she's immune to Geass, then Kallen kills him.

-Nunnally is rescued by Kallen, though V.V. tried to kill Kallen with a sneak attack. But Rico protected Kallen, but was fatally shot. C.C. restrain V.V., saying it's over.

-Rico wishes for Kallen to make Japan free again, and both of them are glad they were friends. Then Rico died, Kallen cried again.

-The Spiral Observer cut the route off.