a person who offers friendship or becomes someone's friend, especially as a formal or deliberate act of kindness.
From befriend (Old English 'be-' + 'friend') plus '-er' suffix meaning 'one who does.' The word has been used since at least the 1800s to describe people who deliberately cultivate friendships.
Befriender is most famous as the name of British telephone support volunteers who call lonely or struggling people—it turns a verb into an identity, marking someone whose whole purpose is offering friendly connection.
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