David
Psalm 89:28ESV·traditional attribution

My steadfast love I will keep for him forever, and my covenant will stand firm for him.

John Calvin Reformed @genevareformer

PSALM 89 The prophet who wrote this psalm, whoever he was, in approaching the throne of grace to make supplication to God in behalf of the afflicted Church, lays down, as an encouragement both to himself and the rest of the faithful to cherish good hope, the covenant which God had made with David.

Commenting on Psalm 89:1-52

C.H. Spurgeon Reformed Baptist @princeofpreachers

My mercy will I keep for him for evermore. The kings of David's line needed mercy, and mercy prevented their house from utterly perishing until the Son of Mary came. He needs no mercy for himself, but he is a representative man, and the mercy of God is required for those who are in him: for such mercy is kept for ever. And my covenant shall stand fast with him.

Matthew Henry Presbyterian @wholebiblehenry

The covenant God made with David and his seed was mentioned before (Psa 89:3, Psa 89:4); but in these verses it is enlarged upon, and pleaded with God, for favour to the royal family, now almost sunk and ruined; yet certainly it looks at Christ, and has its accomplishment in him much more than in David; nay, some passages here are scarcely applicable at all...

Commenting on Psalm 89:19-37