גּאל ga'al
This verb speaks of the duties and responsibilities performed by the senior male relative:
buying back a field which was sold by a poor relative (Lev 25:25 ff)
redeeming a relative who been sold into slavery (Lev 25:48ff)
the "blood avenger" who was to avenge a murdered family member (cf. Num 35:12ff).
From these ideas comes the frequent notion that God is "redeemer" of the weak and powerless.

The participle go'el is used like a noun meaning such a relative.

In Ruth, Boaz makes a connection between this role and the duty to marry a brother's childless widow in order to provide children for the brother (a custom known as "levirate" marriage).