But there is a friend who sticks closer than a brother.
“Blood brother” can refer to people related by birth or to people related by a promise. In antiquity, that covenant was ratified by the parties nicking a hand or finger with a knife, then pressing their wounds together—thus “mixing” their blood and making them “blood brothers.”
In the Old Testament, blood was more often the blood of a sacrificial animal slain when a covenant of friendship was made (Genesis 15). David and Jonathan (the son of King Saul) were covenant brothers, united by their willingness to shed their own blood, if needed, for one another—symbolized by the exchange of weapons (1 Samuel 18:1-4). Jonathan and David are a perfect example of the truth of Proverbs 18:24. Their friendship was closer than that of birth brothers. Jonathan demonstrated his loyalty to David (1 Samuel 19:1) and David reciprocated the same loyalty to Jonathan’s son after Jonathan died (2 Samuel 9:1-2). And Jesus demonstrated that same loyalty to His disciples (John 15:13-15).
Loyalty is a rare trait these days. If you are going to be a friend, be a loyal friend who “sticks closer than a brother [or sister].”
There is no better proof of friendship than to help our friends with their burdens.