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>>133639217While 弟分 can be used for what you described, the most accurate use in Sonic and Tails relationship is that of "sworn little brother".
In Japan, you don't use 弟分 for a junior, you use 後輩, the literal opposite of 先輩. 弟分 is used in situation where the senior and junior demand a closer relationship, more akin to that of siblings, thus the 弟分 refers to the senior as 兄さん (あにさん), which is "big brother". Cases of this, are for example, disciples of a particular art shool, but not regular office or school relationship, unless both parties do feel extremely close... again, as if they were brothers. In popular media, gang members are often depicted as having these types of sibling-like relationships too.
Regarding the Sonic and Tails relationship, Sonic considers Tails a very close friend (as shown in the way they interact in all subsequent games), and the Japs consider that closeness as Sonic seen Tails as a sort of little brother.