*looks at @frenchie-sottises Knack designs: this, this, and this*
The idea of Knack being an entity powered by relics than made from them sounds so interesting. Everything about relics is already a huge mystery other than they can be found in structures that are usually guarded by Guardians and can power various machinery. The Hidden Land in the first game raises even more questions.
My own iteration on this idea would share some similarities to Spectrobes. The collectible creatures, who also share the same name, actually start as fossils. You have to excavate them first and awaken the sleeping Spectrobe with your voice.
Doctor Vargas finds a fossilized Knack on the pedestal. His chest orb is dormant but flashes when the younger scientist reaches out as if accepting to be taken. It takes a few years for Knack to awaken since energy is still needed than just a vocal command. His body is similar to living stone like with the trolls from Trollhunters so he is quite durable.
Relic energy affects his growth and can be gathered by touching relics. Dispersing it or taking too much damage will make Knack shrink. He's still mute when little as his vocal cords aren't developed enough for usage. Knack can grunt or roar though but not talk.
When I do draw my own version, there will be added details than just his normal canon design. Knack gains fur and thick spikes once he surpasses 10 ft. He'll have a tail that becomes similar to an ankylosaurus. Knack also has a scaly hide which is a mix between beetle chitin, fish and reptile scales.
Can't forget hooked claws since he tends to climb a lot. Basically a chimera version of Godzilla that can become a full blown Kaiju. As for the element and gem transformations, they are adaptations than armor. Each shifts temporarily shifts his form to suit the item in question. It's easy to see what material Knack used as it is more prominent.
Simplified Examples
Ice: Thick blue fur and clusters of ice on his back
Wood: Grassy mane with wooden quills
Metal: Prickly metallic hide paired alongside serrated blades
Stealth: Chameleon hide coated in reflective scales
Guardian: Humanoid with all inhuman traits as tribal armor
Dark, Brittle, Sunstone, Massive, Diamond, Vampire, Speed, Power, Saber and Shooting would operate a bit differently. They are more like enhancements similar to weapon parts from Spectrobes. Boost some stats and weaken others with physical changes such as Harumitey with its defensive Lazos part here. 👇
There is a bit more worldbuilding but I'll save that for another post. What do you think so far?