Jester Thomas is a white phantom available for summoning at Earthen Peak area. His sign is located to the right of two Grave Wardens outside the corridor to Mytha, the Baneful Queen's fog gate. An odd figure dressed in jester's clothing, he dual-wields two Pyromancy Flames and is capable of casting damaging fireballs, making him an excellent ranged ally against Mytha.
He will often pull Mytha's attention away from the player, fully exposing her back to an assault. However, it is advised the poison pit in her arena is drained by burning the windmill blades in Central Earthen Peak or by using Pharros' Lockstones before the battle, as it will impede his movement and slowly drain his health, reducing his effectiveness.
His armor, the Jester's Set, can be bought from Magerold of Lanafir in the Iron Keep, close to that area's first bonfire. All four pieces grant a specific bonus to the player (wearing the Jester's Gloves, for example, increases the amount of souls acquired from fallen enemies).
Jester Thomas returns in the Crown of the Sunken King DLC. He invades the player near the bottom of Dragon's Sanctum, right inside from the Lair of the Imperfect. As soon as Jester Thomas invades the player, he will do the "This one's me" gesture once he sees the player, and if the player dies, he may do the "Mock" gesture. His arsenal contains Flame Swathe, Forbidden Sun, Warmth, Flash Sweat, Firestorm and Great Fireball.
Jester Thomas can be a powerful opponent when confronted as a dark spirit, as he will always dodge the player's attacks and quickly cast his pyromancies. However, as with most spell-casting NPCs, he'll often leave himself open to attack while casting, and dealing enough Poise damage to him as he is readying a spell will interrupt it. A follow-up attack while he is recovering from stagger is not recommended, as he'll most probably roll away and retaliate.
As virtually all of Jester Thomas's damage output is through his pyromancies, equipping fire-resistant gear such as Flame Quartz Ring and utilizing spells such as Flash Sweat will greatly reduce the amount of damage he does. As well, a small basin of water at the fog gate where he invades can be rolled into to gain the Waterlogged status effect, further reducing the damage that his spells deal.
When half of his health has been depleted, he'll cast Warmth to heal himself. Run toward this spell's area of effect, attack Thomas to make him run away and claim the spell as yours, effectively stealing its healing properties for yourself for the rest of the battle.
Profound Still will stop him from using any pyromancies, and he'll switch to a sword in response.