C. F. Keil and Franz Delitzsch on Psalms 25:21

Psa 25:21

Devoutness that fills the whole man, that is not merely half-hearted and hypocritical, is called תּם; and uprightness that follows the will of God without any bypaths and forbidden ways is called ישׁר. These two radical virtues (cf. Job 1:1) he desires to have as his guardians on his way which is perilous not only by reason of outward foes, but also on account of his own sinfulness. These custodians are not to let him pass out of their sight, lest he should be taken away from them (cf. Psa 40:12; Pro 20:28). He can claim this for himself, for the cynosure of his hope is God, from whom proceed תם and ישׁר like good angels.