A.T. Robertson Commentary Romans 8:27

"and he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to [the will of] God." — Romans 8:27 (ASV)

He that searcheth (ο εραυνων). God (1 Samuel 16:7).

According to the will of God (κατα θεον). See 2 Corinthians 7:9–11 for this phrase κατα θεον (according to God). The Holy Spirit is the "other Paraclete" (John 14:16) who pleads God's cause with us as Christ is our Paraclete with the Father (1 John 2:1). But more is true as here, for the Holy Spirit interprets our prayers to God and "makes intercession for us in accord with God's will."