Martin Springer

Martin Springer (マーティン・スプリンガー) was the brother of Connie Springer and a resident in Ragako village. He was transformed into a Titan by the Beast Titan during Wall Rose's supposed breach.

Human
Not much can be made out from his appearance, but it can be seen that he was a small boy who wore a loose shirt with shorts, small shoes, and had medium length hair.

Clash of the Titans arc
Martin Springer is one of the victims gassed by Marleyan soldiers before being turned into a Titan by Zeke Yeager at the start of Wall Rose's Titan invasion.

Martin Springer is revealed for the first time during a flashback. When Connie arrives to Ragako looking for survivors, he recalls a time when he had a happy family. Connie looks for his family heavily worried with no result. It is later revealed that Martin is one of the Titans that were created by the Beast Titan after the events of Eren and Annie's brawl in Stohess District. His Titan is eventually killed at some point before Wall Rose was declared Titan-free.