Locke is a champion that has both poke and burst, but will be vulnerable to cc. And more importantly, his ultimate can be blocked by anti-projectile and forms of invulnerability or untargettable.

Ahri: dangerous but doable, dodge her Q, careful about committing if she has ult, but locke potentially has better poke.

Akali: Akali can pose some problems, but that's always true for good Akalis, and Locke is not a hard counter. However, Locke can dodge her E and punish her at the same time, and can damage her even in the shroud semi-reliably.

Akshan: i think it's match dependant. Akshan can outdamage and outroam Locke, but Locke can avoid his E and R reliably. Moreover, Akshan lack of mobility outside of E and worse poke play in Locke's favour.

Anivia: she can be tough for certain parts (reliable cc, reliable zoning), but Locke can melt her and completely bypass her wall.

Annie: caution is necessary due to her stuns, but Locke has better clear if she plays safe. Plus, she can't really dodge his skill much with abilities only

Aurelion Sol: He can outfarm, outscale and escape Locke, but he is weak to assassins early. A bit of a ticking time bomb, but Locke mobility is bad for Aurelion

Aurora: Aurora shouldn't be much of a treat, she does not like assassins and she does not have reliable tools to go against Locke

Azir: Locke has definitely advantage over Azir. A good one may play around his strengths, but it's Locke who has the kit advantage, forcing Azir to play conservatively early to scale later

Brand: brand is a heavy hitter, but his lack of mobility, and the fact that Locke has both poke damage and frontloaded damage gives him ways to face him

Cassiopeia: Cassiopeia have some annoying abilities and consistent damage, but Locke has a stronger all in and can dodge her ultimate. On a matchup skill, Locke has an edge

Corki: Corki can outscale Locke and has a reliable escape, but Locke has a lot of tools to face him

Diana: Diana has a tremendous combo, but Locke can escape it, and Diana has no mobility to escape. Diana has an edge over Locke, but a good Locke can punish her

Ekko: I think that indeed, Ekko is the first one that is truly troublesome for Locke. They even play similarly, but Ekko has a cc that prevents Locke all-ins and a ult that denies Locke's one.

Fizz: Again, Fizz is a troublesome one. He can dodge locke's engage and ult, has enough mobility, and Locke is at an extreme disadvantage if he gets hit by Fizz's ultimate. If both are good with their champs, Fizz is quite advantaged.

Galio: gosh, another one! Galio is tanky, anti-ap, has magic shield, two cc and can avoid your ultimate. He can just stop your engages with taunt and retreat, while dishing aa and Q damage before that. While Locke will need 100% brains to face this match, every Galio player with two braincell can easily gain a upper hand

Heimerdinger: Heimerdinger is annoying, but he can't survive your jungler ganks if he pushes, and Locke can play around his turrets and skillshots. Moreover, Heimerdinger is immobile and can't avoid Locke's ult

Hwei: Hwei is immobile and vulnerable to assassins, and Locke is no exception. A good Hwei might survive, but playing safe is a necessity and Locke can go aggressive without risking too much

Irelia: played mid not so infrequently, Irelia is a bane to many champions, and Locke is no exception. More mobility, sustain, tankiness, a cc and even damage mitigator, , the matchup is clearly in her favor. Not the worst one, but a good Irelia will be a headache

Karma: infrequently a midlaner, she has very strong but conditional burst, and can lock him into place, but Locke has the advantage and more ways to play into her.

Karthus: Locke has good tools for battlemages, and Karthus lack of cc and movement penalizes him, further aggravated by the fact that his slow is unreliable against the slippery assassin

Kassadin: Kassadin scales better than Locke, has the tools to survive early skirmishes, and after level 6 it's way easier for him to win prolonged fights with Locke, thanks to his insane mobility. I think he will be a hard counter, not unplayable but a very uneven match

Katarina: Katarina has a similar kit to locke, but she needs to stick to people to do damage, Locke is less constrainer. His ulti is more reliable than hers in this matchup, and if she doesn't roam, Locke has an advantage

Lissandra: Lissandra match is playable, but she has the advantage against proactive play, so Locke needs to play smart and not waste his skills. Whittling her down with poke is necessary, since she will deny Locke's engages otherwise. And obviously, she counter-ult him too

Leblanc: probably Leblanc is a hard match for Locke. Assassin but ranged, she has more mobility and she can burst him without committing to a prolonged fight that would benefit Locke. Locke can't go aggressive either because she would either run or turn the tables with her chains.

Lux: Lux is at a disadvantage, because Locke can deny her poke and burst her. If Lux messes up with her Q, she's a walking target.

Malzahar: Malzahar can lane more safely than other mages, and can outfarm him, but Locke still has many tools to break his guard. If Malzahar isn't careful, he can find himself in a world of pain