Who is the who in Romans 8:27?
And he who searches hearts knows what is the mind of the Spirit, because the Spirit intercedes for the saints according to the will of God.
Is it God? or is it the man who has the Spirit?
According to 1 Sam 16:7, 1 Kings 8:29, Ps 7:10, Prov 15:11, Jer 17:10 & Acts 1:24, it is the LORD (YHWH in OT, and Lord = kyrios in NT) who searches the heart. Thus, the phrase, "he who searches hearts" had become a phrase denoting God.
"Who" is not in the Greek of Rom 8:27. I would translate it literally as, "But the one searching the hearts [plural] knows the what [is] the mind of the Spirit … " Thus, whoever is searching does so for all hearts.
Several translations (NLT, NRSV, CEV) actually insert either "God" or "Father" to make this connection clear. Thus, we have a picture of God (either just the Father or the entire Godhead, not specified) searching our hearts in accord with the mind of Spirit; who in turn, intercedes for us with God. Do not ask me to explain it, it is just what is written there!
Only God has the ability to "know what is the mind of the Spirit," so "the man that has the Spirit" (the person indwelled by the Holy Spirit) wouldn't seem to make sense, even though the Spirit's words aren't expressed in the verse; God the Father knows precisely what the Spirit is thinking, which is interesting in the sense of the intimacy within The Trinity, itself, and the omniscience of the Father.
Fascinating to realize, too, that Jesus Christ is perpetually interceding on behalf for believers in the presence of God, according to verse 34 of Romans 8, and also referenced in Hebrews 7:25!