Mage

A Mage as ally is the wizard NPC the party hired, the academy contact who agreed to help on this one job, or the older mentor who finally showed up. The Mage has opinions about magical correctness and will share them at length. The party should feel that they are working with a peer, not a vending machine, and the Mage should refuse one request per session on aesthetic or ethical grounds.

In a fight on the party's side, the Mage's job is to remove a problem the party cannot. Counterspell on the enemy lich's Disintegrate is exactly the play. Save Cone of Cold and Fireball for moments when the cleric specifically asks for help, because nothing teaches party caster discipline like seeing a 9d6 Fireball drop a wave of cultists from across the room. Misty Step keeps the Mage out of melee, where 81 HP and AC 15 will not last long against a serious enemy.

Out of combat the Mage is the answer to identifying magic items, sending Sendings (which the Mage does not have, so the party still has to find a temple), and translating the language the campaign forgot to teach a player. Use the Mage to give the party information that closes loops the players forgot to tug on. Detect Magic at-will means the Mage notices enchantments the players walked past two sessions ago.

Have the Mage be slightly contemptuous of the cleric. Wizards and clerics rarely get along, and a low-key rivalry between two NPCs the party works with is more interesting than another faction war.

A Mage is the generic-wizard NPC the SRD gives you to drop into any cult, tower, library, or noble retinue. CR 6, 81 HP, AC 15 from Mage Armor, save DC 14, and a spell list that does the three jobs an enemy caster needs to do: artillery, escape, and counterplay. Treat the Mage as a turret with feet, not a melee threat.

Open with Fireball at level 4. That is 9d6 fire averaging 31, DC 14 Dex save, 20-foot radius at 150 ft. range, dropped on the largest cluster of PCs you can find. The Mage has it twice per day, so you can lead with it and still have one in reserve for round three. After the opener switch to Multiattack with three Arcane Bursts at +6 for 16 force damage each, ranged or melee. The 120-foot range means the Mage never has to walk into a sword.

Misty Step (3/Day bonus action) is the kiting tool. Round one Fireball, round two Multiattack, round three Misty Step out of the rogue's reach and Multiattack again. Save Cone of Cold (1/Day) for the moment the party finally collapses on the Mage at melee range. 8d8 cold in a 60-foot cone clears the room. Save Invisibility for an emergency escape, or burn it on a friendly NPC to make the next scene a stealth chase. Fly is the build-an-encounter spell: cast it before initiative if the Mage knows the fight is coming, and the party is suddenly fighting an artillery piece they cannot reach.

Protective Magic is the difficulty knob. Counterspell or Shield, 3/Day, as a reaction. Use Counterspell on the first big save-or-suck the players throw (Banishment, Hold Person, Hypnotic Pattern), and use Shield to negate the rogue's Sneak Attack the round they finally close. Once Protective Magic is gone, the Mage is fragile, and a smart party will burn through it on purpose to make the wizard squishy.

Give the Mage a name, a robe color, and a single weird affectation. "The Mage casts Fireball" is forgettable. "Velora Threefingers stops humming, looks at the rogue, and casts Fireball" is a recurring villain.