Plane of Origin: Kamigawa
First Planeswalk: Lorwyn/Shadowmoor
Appearance: Sezashi is an Orochi, a horned snake with four arms, in his forties. Mostly green, with black patterning on his scales. He's probably about 24ft long from the tip of his tail to his snout, and generally stands a bit over 6ft off the ground, though it varies a bit depending on his positioning. He has two matching ornaments of different colors wrapped around the base of his horns. Four floating objects typically float around him, representing his bond to a Kami. Two of them are seeds of some kind, from full fruits to emerging sprouts. The two others are two-dimensional moving symbols, one with movement reminiscent of waves, and the other of winds and clouds. The last two always appear to be facing those that observe them, no matter how many people do or where they are.
Backstory: Sezashi has served for over a decade as a Merge-Keeper on Kamigawa, guarding one of the passages between Kamigawa's spirit realm and mortal world, and welcoming the Kami entering the world through it. Bonded and working with Adima, a Kami of Twin Worlds, they shared a vision of a future of the two worlds being reborn as one, better than the sum of its parts.
Sezashi, similarly to some in the more activist Order of Jukai, had visions about the end of the Kakuriyo brought on by technology, but his interpretation is very different. He sees it as bringing forward this new world, and the end of the Spirit realm only being the beginning of something new, and something wonderful, his and Adima's dream.
As such, when an Imperially-backed Futurist approached them to test an upgrade and prototype of a new Merge gate, aiming at better controlling the flow between the Realms and being able to regulate them, Adima and Sezashi accepted the one they guarded to be used for the test.
The Order of Jukai didn't take kindly to this new technology they didn't work along, messing with the path to the spirit realm. They organized an attack on the gate to destroy the prototype while Adima and Sezashi were distracted by a newly arrived Kami. Detonating a bomb filled with spiritual energy near a hole in the world to the spirit realm stabilized by untested technology might not have been the best idea.
The resulting rift spit out confused and angry kami, threatened to destroy part of the forest, and of spreading dangerously, damaging both worlds. After hours of running around trying to fix the consequences for both of them, Adima resolved to patch the rift, and incidentally close the Merge, something that isn't typically thought to be possible. But to do so, Adima had Sezashi channel them into the merge, which consumed them, severing their bond with Sezashi, and they've not been seen again since.
The loss of his closest friend, mentor and purpose in a already harrowing day was enough to ignite Sezashi's spark, sending them to another world, one that resulted of the fusion of two linked parts.
That was a couple years ago. Since then, he's traveled the worlds in search of worlds like his own, and of a way to bring back Adima. Despite their bond being broken, Sezashi still has access to the magic they granted him, unchanged, which he takes to indicate there's some part of Adima still existing somewhere, perhaps inside Sezashi himself, perhaps lost in the Spirit Realm, which could be used to bring them back.
Magic, gear and/or abilities: Their main magic is one of duality, one focused on two. The four elements floating around Sezashi are "charges" he can use to cast a variety of spells, doubling or halving physical characteristics of himself or other people and objects. The simplest may be size, but those spells can affects things in many different ways, from doubling something in its entirety, creating a clone of them or himself (or halving, he often halves the number of arms he has to be less conspicuous on planes without four-armed nagas), to halving or doubling something's weight or even thoughts (though less physical properties such as thoughts are harder to affect).
On himself, he can strengthen those spells by expending more "charges" on it, spending two, three or four charges on the same thing to multiply or divide the chosen property by 4, 8 or 16 instead of two. He cannot stack like this quite as well on others or objects (sorta, his clothes and gears count as "him" as far as his magic is concerned, for obvious reasons.)
His magic lasts as long as he wishes (and is relatively near), but he loses that charge until he ends the effect it is associated with. The element floating around him have to physically travel to whatever they'll affect, so it isn't always the fastest process either. They disappear while sustaining a spell. He can't in any circumstances maintain more than four spells that way (he could with direct help from Adima, but that's not really on the table anymore.)
As far as gear, he carries for a main weapon a metallic staff of sorts that can split off into two batons (he prefers keeping two arms free but occasionally can double that into four). The deployment and separation is entirely mechanical, so it works off Kamigawa. In truth, its tips can produce energy blades to turn it into spears/daggers, but that doesn't work on most planes and a solid metal staff is enough of a weapon for him anyway.