The first thing that you need to know: pyke isn't a properly mid laner. What it means? It's a little bit hard to make it usable. Knowing that, here we start.

Pyke has a specific game style if compared with the others champions. In the first look, it apears to be a fragile assasin, being impossible to survive against explosive damage. But here's a secret: you can be a optimal "tankssasin". The ultimate prevides the ideal capacity to literally ignore shields, so you can build an off-tank item. That's the major point of playing pyke as mid laner.

1º: Play roaming.
Pyke has good skills for mobility, so here is the trick. You can abuse this advantage (espeacilly against mages) to "visit" the bot lane many times as you can. Literally, after level 6, you can dive and destroy the enemy (carefull with soraka and morgana)

Pyke's passive is extremely powerfull, if you know how to use. Here comes the point of when you stay or leave: beaing a champion who doesn't stacks health, you NEED to get this. For example, if the enemy is a tank, you can stay more in the fight, even if your health is low. Ik potencial, in the other hand, need you to stay away most as you can. Is just question of play the game. Naturally you get it.

The differential of my guide: my build. It's look a bit strange, but i can afirm that works.
Here come how to build:
Start- Long sword and 3 potions.
1- Stay in the lane until you get 1050G. Get a serrated dirk and a boot.
2- At this point you probally would be level 6, so you can start roaming. In the next back, buy the mobility boots, a control ward and the oracle.
3- Finish the draktharr.
4- From here, literally just build the itens in order from the common build (yoummuus, essence reaver, etc)
Alert: Where´s the boot in the final build?
You will exchange it for the dead Man's Plate.

The arsenal: {"title":"Pyke itens","associatedMaps":[],"associatedChampions":[555],"blocks":[{"items":[{"id":"1036","count":1},{"id":"2003","count":1},{"id":"2140","count":1},{"id":"2138","count":1},{"id":"2139","count":1},{"id":"3117","count":1}],"type":"Inicial e pots"},{"items":[{"id":"6691","count":1},{"id":"3142","count":1},{"id":"3508","count":1},{"id":"6694","count":1},{"id":"6333","count":1},{"id":"3742","count":1}],"type":"Padrão"},{"items":[{"id":"6691","count":1},{"id":"3142","count":1},{"id":"3508","count":1},{"id":"6694","count":1},{"id":"3156","count":1},{"id":"3742","count":1}],"type":"Mage"},{"items":[{"id":"6691","count":1},{"id":"3142","count":1},{"id":"3508","count":1},{"id":"6694","count":1},{"id":"3075","count":1},{"id":"3742","count":1}],"type":"Armor"},{"items":[{"id":"3156","count":1},{"id":"3143","count":1},{"id":"6695","count":1},{"id":"3026","count":1},{"id":"3074","count":1},{"id":"3814","count":1},{"id":"3181","count":1},{"id":"6035","count":1}],"type":"Situacionais"},{"items":[{"id":"6691","count":1},{"id":"3142","count":1},{"id":"3508","count":1},{"id":"3095","count":1},{"id":"3031","count":1},{"id":"3742","count":1}],"type":"Le Build"}]}

When use:
Randuin's Omen: Critical enemy team (crit yi, crit adc, yone and yasuo). Build as the 4ª item.
Thornmail: You can buy the bramble vest after the duskblade if playing against high healing champions (irelia, yone, soraka, sylas).
GA: If becomes a long match, you can exchange the DMP for the GA. After using the passive, just exchange back.
Edge of the night: Good against multiple stun mages, but i don't use it. The Silvermere Dawn is much better.

An OTP pyke player who achived master playing as a mid laner. Here's my OP.GG: https://br.op.gg/summoners/br/Ghost%20Diver