Takeo (Kor. 타키오) was once a member of the DA-5 created by Dr. Aris. Believing that his sister was held captive by the Union, he participated in its experiments and followed orders in order to protect her.

Spoiler

In Chapter 151, however, it is revealed that this is a ruse concocted by Dr. Aris, who created the facade for the sake of her own entertainment. Takeo is later invited to live in Frankenstein's house after betraying the Union and erasing all traces of his and Tao's existence, which resulted in the Union's belief of their failure and death as experiments. Recently, both he and Tao have asked Frankenstein to enhance their powers in order to increase their capability to protect their comrades, especially M-21, with whom they have formed a strong bond of loyalty and friendship. He is a member of the RK-4 and his codename is 'Number Two'.

(Source: Noblesse Wikia)