It was in the Tokyo base that Lloyd Asplund, with the help of Cecile Croomy, began to examine the Guren in great detail. It wasn't long until it was outfitted with numerous special enhancements.
During the battle to take over Tokyo with the Black Knights. Kallen was rescued from her imprisonment and she was able to steal the Guren SEITEN for her escape. The original Guren creator Rakshata Chawla was enraged to see the alterations done to her designs and recognized who was responsible for the work. With all these advancements at her command. Kallen was able to easily defeat any other KMF that stood in her way. The radiation arm had been enhanced to fire a much powerful beam and the wings provided incredible speed. She arrived just in time to save Lelouch.
In the fight with the Lancelot Conquista. Kallen in the Guren SEITEN easily won. She literally chopped this Lancelot into pieces. Before she could deliver the killing blow, the Lancelot pilot, Suzaku, evaded the attack and due to the influence of his "Live On" Geass effect. He launch a powerful F.L.E.I.J.A. bomb that destroyed much of Tokyo and was reported to of killed one million people.