Well this thing is nightmare fuel....but let's get into it! (This is my first time reviewing a champion like this, so bear with me please ^^)
Design-wise, it's pretty terrifying. I'm sure that an actual rendering wouldn't look exactly like the picture, but the point remains that SALF should look like a skeletal/emaciated creature. Possibly a result of tampering with necromancy? Not necessarily making him an undead, but if it came from a place like Zaun, they could have combined their science with magic to make this thing possible. Wouldn't be the first time.
We do have an artificially created life form in the form of 
, but that doesn't necessarily mean we can't have another one. In fact, that might leave it open to some very interesting possibilities for lore. Perhaps this creature is like Zac 2.0 after Zac's parents took him away from the experiment that was meant to turn him into a weapon? Something interesting to work with. I notice you don't have a lore here so I assume that's up for interpretation.
Onto his kit....so from what it looks like, he's a Support/Fighter champion who could do well in either a solo or duo lane. His passive is definitely what'd make him a pretty fearsome support. It'd be a little odd once laning phase is over since it only effects your closest ally and in a team fight, that can change pretty quickly. So you'd have to either stand back with your ADC or head up front with your other damage dealers. And since it's an AD boost, AP champs don't synergize well with it. I have a question about his Secret Evolution passive, though; the +10 bonus damage when you max out an ability (so a total of +40 AD when you're level 18), does that go to your ally via Symbiosis as well or is it still 10% of your total AD? Either way, this is a very interesting passive. It's easy to see it as OP, but if you're playing support, then you're not going to be building any AD until your third or fourth item anyway, so it's not a very big boost. And in solo lane, it's not going to be effective until you max an ability first - the earliest of which you'd be doing is most likely around level 8 if you rush one ability above the others. I'd be careful about that, since it might give an unfair advantage, but if the champion's kit is design to put points into all your abilities evenly for maximum effectiveness, that would mitigate that problem. It helps that it looks like the rest of his abilities have low scaling, though....
His Q seems pretty standard. A damaging ability that fits well with his role and purpose, and with Symbiosis active it'd be a good way to force enemies out of lane. It sounds like he'd do well in a bully duo lane with someone like Sivir or Draven. My main concern with this would be the possibility of giving your Symbiosis partner a little too much power early on, but then again, I'm not good with numbers, so it could be more fair than I think it might be lol. It has a relatively low mana cost and an average cooldown, so I imagine it's possible to spam it with some CDR. (Also, I should point out you put "Maman" instead of "Mana" under his Q...XD)
Now his W...this is what will probably make him OP if it's not balanced correctly. At max rank, this thing is gonna hurt. A lot. Especially if you're building pure AD. With the sustain it brings with the more enemies you hit, coupled with something like 
and 
is gonna heal for a lot. But the fact that it heals your ally for 85% of the damage dealt at max rank, that has HUGE potential to be hard to play against. Grievous wounds can only do so much, after all. This thing would be a nightmare in team fights, especially if it decided to build a little bulkier. This is the ability I'm most unsure about, since it's an AoE and a heal, not only for you but for a teammate too. One thing I might do is to bump up the mana cost a bit, just to make it one of those things you try not to use too much and possibly reduce the healing on your ally. Also, not sure you should be healing that much from minions either. I'd either cut that entirely or lower those numbers a bit. Since this doesn't say anything about healing from jungle monsters, I'm going to assume it doesn't, otherwise he'd make an interesting jungler.
His E is honestly the least impressive thing about him, but it's still a very useful thing to have. An immunity to slows is always a very nice thing (bye-bye perma-slow 
!) and giving that to an ADC would make it a lot easier to escape from a bad situation. Honestly, you could probably lower the mana cost on this a bit, but keep the cooldown. Now a 4 sec immunity to slow might seem OP, but there are worse things already around (I'm looking at you, 
so I don't think it's too bad at all.
Now his R honestly has me the most confused. Since I've been assuming this champion is built more for support, it's a little odd to have an alternate form that's definitely not a support role. That's not to say that's a bad thing, but it just feels like it conflicts a bit with the rest of his kit. So just judging from the numbers, it sounds like he changes gears from a support-like role into something that I'd consider a pseudo-juggernaut. With his passive pretty much ignored now, he has less to play with, but his abilities hit harder than ever and I'm a bit worried that, like with his W before, this could make him seriously OP. His saving grace might be that he would likely be easily kite-able in this form, though his E might mitigate that a bit. It's good that Symbiosis isn't in effect for this form, though, as that would definitely be super broken. I think that the only thing that might need some looking out for is numbers here. We want to make sure he has some counter play, after all.
So overall, I think this is an interesting champion design with a handful of small flaws that could be ironed out. It's terrifying, yet also helpful. It might be a little on the stronger side of the spectrum, but I don't believe it's truly broken. There's still plenty of room to get rid of this guy (I feel like early solo lane for him will be hellish) but he could also be oppressive if someone knows what they're doing. The main thing I think would need to be adjusted slightly are some of the numbers, but like I said before, that's not necessarily my strong suit, so take that with a grain of salt. Regardless, this is a pretty neat champion design! Keep them coming and I'll see about looking at a few more of them when I can!
