In the New Testament, the Holy Spirit is given as ‘”another (i.e. a second) Paraclete” (Jn. 14:16), taking over Jesus’ role as counsellor, helper, strengthener, supporter, adviser, advocate, ally (for the Gk. paraclētos means all of these).’

The personality of the Holy Spirit is clear: ‘Like the Father and the Son, he acts as only a person can do—he hears, speaks, convinces, testifies, shows, leads, guides, teaches, prompts speech, commands, forbids, desires, helps, intercedes with groans (Jn. 14:26; 15:26; 16:7–15; Acts 2:4; 8:29; 13:2; 16:6–7; 21:11; Rom. 8:14, 16; 26–27; Gal. 4:6; 5:17–18; Heb. 3:7; 10:15; 1 Pet. 1:11; Rev. 2:7, etc.). Again like the Father and the Son, he can be personally insulted (blasphemed, Mt. 12:31–32; lied to, Acts 5:3; resisted, Acts 7:51; grieved by sin, Eph. 4:30).’

Specifically, the Spirit:-

1. reveals Jesus’ reality and the truth about him, first by reminding and further instructing the apostles (Jn. 14:26; 16:13; Eph. 3:2–6; 1 Tim. 4:1) and then by so enlightening others that they receive the apostolic witness with understanding, confess the divine Lordship of the man Jesus, and experience his life-changing power through faith (Jn. 16:8–11; Acts 10:44–48; 1 Cor. 2:14–16; 12:3; 2 Cor. 3:4–4:6; Eph. 1:17–20; 3:14–19; 1 Jn. 2:20, 27; 4:1–3; 5:6–12);
2. unites believers to Christ in regenerative, life-giving co-resurrection, so that they become sharers in his kingdom (cf. Rom. 14:17) and members (living limbs) in the body of which he is head (Jn. 3:5–8; Rom. 6:3–11 with 7:4–6; 8:9–11; 1 Cor. 6:17–19; 12:12–13; Gal. 3:14 with 26–29; Eph. 2:1–10 with 4:3–16; Tit. 3:4–7);
3. assures believers that they are children and heirs of God, both through the direct witness of immediate inward certainty and by the indirect testimony of implanted filial instincts and dispositions, from which the reality of the new life may safely be inferred (Rom. 8:12–17; 2 Cor. 1:22; Gal. 4:6; Eph. 1:13; 1 Jn. 3:24; 4:13; 5:7);
4. mediates fellowship with the Father and the Son of a kind that is already heaven’s life begun, and is thus, as a first instalment, a guarantee of the fullness of heaven’s life to come (Rom. 5:5; 8:23; 2 Cor. 5:5; Eph 1:14; 2:18; 4:30; 1 Jn. 1:3 with 3:1–10, 24);
5. transforms believers progressively through prayer and conflict with sin into Christ’s moral and spiritual likeness (2 Cor. 3:18; Gal. 5:16–25; Jude 20–21);
6. gives gifts—that is, witnessing and serving abilities—for expressing Christ in the believing community that is his body, so building it up (Rom. 12:3–13; 1 Cor. 12; Eph. 4:7–16; 1 Pet. 4:10–11), and for evangelistic proclamation in the world, so extending the church (Acts 4:8, 31, 9:31; Eph. 6:18–20);
7. prays effectively in and for believers in Christ who feel unable to pray properly for themselves (Rom. 8:26–27);
8. prompts missionary action to make Christ known (Acts 8:29; 13:2; 16:6–10), and pastoral decision for consolidating Christ’s church (Acts 15:28).

New Dictionary of Theology, art. ‘Holy Spirit’ (slightly reformatted, emphasis added)