why fight on 99 and 100, he was close, others didn even know how many brothers Duryodhan had. Yuyutsu is not something common, i doubt many know about him,
He mved to pandavas side in the beginning of the war,
Vikarna was another blood brother of Duryodhan who was symphatic to Pandavas but he chose to remain with kauravas and was killed by Bhima in the end.
Yuyutsu's character is close to what Bibhishan did in Ramayana, he was made king of Indraprasta post the war as he was technically a brother to pandavas. even the son of Karna was made a king of a smaller state as technically he was the son of eldest pandava.
most of these are not shown in the tv series Mahabharat, including the story of Barbarik who is the grandson of Bhima who was not allowed to take part in the war by Krishna as he was considered most powerful of all.