Bartholomew, the Leopard Gecko Rune Knight

Giant’s Might (3/Day). Bartholomew has learned to imbue himself with the might of giants and becomes large for 1 minute. As a result of this, his Strength score increases to 18, and his Constitution score increases to 12, he has advantage on Strength checks and Strength saving throws, his Armor Class decreases to 16, and his Stealth bonus decreases to +6.

Rune Knight Magic. Barthomew has many magical runes engraved on his scales and can invoke their power during combat. Only one rune can be invoked every turn, but ongoing effects remain, even if other runes are invoked afterward. Each rune can only be used a limited number of times per day and needs to recharge after each individual usage. The type of rune restored at the beginning of every turn is determined with a d6 roll.
Using runes does not take an action.
Rune magic is not equivalent to spellcasting and does not follow it's rules.

Fire Rune (Max 3/Day, Recharge 1). When using his tail slam on his turn, Bartholomew can invoke a Fire Rune to restrain all targets in flaming shackles. In addition to the normal effects of the attack, all targets take 7 (2d6) fire damage and must succeed on a STR saving throw (DC:14) or be restrained for 1 minute. While restrained by the shackles, the target takes 7 (2d6) fire damage at the start of each of its turns. The target can repeat the saving throw at the end of each of its turns, banishing the shackles on a success.

Frost Rune (Max 2/Day, Recharge 2). At the start of his turn, Bartholomew can invoke a frost rune to increase his sturdiness. He gains a +5 bonus to Strength and Constitution checks and saving throws for 2 turns, which includes his grappling DC (20 -> 25).

Hill Rune (Max 2/Day, Recharge 3).  At the start of his turn, Bartholomew can invoke a hill rune to bolster his defense, granting him resistance to bludgeoning, piercing, and slashing damage for 2 turns.

Storm Rune (Max 3/Day, Recharge 4-5). When making a defensive snap at a creature entering Barthomews reach, he can invoke a storm rune, making him hyper-aware of his surroundings, causing the next attack roll targeted at him to have  disadvantage, as well as one of the following effects, decided by rolling a d6:
1-2: The next two bite attacks have advantage, even if the target is not grappled.
3-4: The next two grapple targets have disadvantage on the escape roll.
5-6: All targets of the next tail swipe or tail slam have disadvantage on all saving throws of the attack.

Runic Shield (Max 3/Day, Recharge 6). If the Bartholomew is hit by an attack, he can cuse the attack to miss instead. If he fails a saving throw, he can choose to succeed instead.

Rune Surge (1/Battle). Bartholomew pushes his rune magic beyond it's limits for a moment, allowing the usage of two runes on the same turn.

Storm Rune Seeing. Bartholomew can glimpse into the future and can't be surprised as long as he isn't incapacitated.

Gecko Cling. Bartholomew can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. Bartholomew either grapples and makes two bite attacks or one tail swipe and one bite attack.

Grapple (Large size only). Bartholomew grapples up to two targets with his sticky front claws. (escape DC 20).

Bite (Large size only). Melee Attack: +8 to hit, reach 10 ft., Hit: 12 (2d6+5) piercing damage. If the target is grappled, Bartholomew can make this attack with advantage.

Tail Swipe (Large size only). Bartholomew swipes his tail in a 15ft. cone. All creatures in the area must make a DC 19 Dexterity, or DC 16 Strength saving throw or be knocked prone.

Tail Slam (Large size only). Bartholomew slams his tail in a spot up to 10 ft. away from him. Every other creature in a 15 ft. radius from that point takes 7 (1d6+4) bludgeoning damage, is pushed away 10ft. if their size is medium or smaller and must succeed on a DC 14 Dexterity, or DC 12 Strength saving throw or be knocked prone.

Opportunity Attack. When a creature leaves Bartolomews reach (10ft.), he can make a bite attack at, or attempt to grapple that creature, if he isn't already grappling two other targets.

Defensive Snap. When a creature enters Bartolomews reach (10ft.), he can make a bite attack at that creature.

Bartholomew can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Bartholomew regains spent legendary actions at the start of his turn.

Bite. Bartholomew makes one bite attack.

Tail Slam (Costs 2 Actions). Bartholomew makes one tail slam.

Jump (Costs 2 Actions). Bartholomew jumps up to 20ft. without provoking opportunity attacks.

A squire situation (1/Day, when hit points below 50%, Costs 3 Actions). Barholomew calls in his squire Quentin to assist in battle, who remains until dismissed with a bonus action.