Scratch my back and ill scratch yours.

You've made a pact with a low level entity. In exchange for the power to summon then on the Material plane you promise to help it gain power and move up it's own ranks.

Summoning

At level 1 the pact you've made allows you to summon your patron in their full strength.As you learn to channel your powers and do your patrons bidding on the Material world they grow in strength and move up their own ladders. In this form you and your patron split damage between each other, if their avatar is slain you lose any resistances and benefits they were granting you. 

After a 10 minute ritual is done you can summon your patron using the Sidekick rules from Tasha's with the exception that it has fire resist. It lags behind you and the party by 1 level except the first. (I.E Party Level = 1, PAtron Level = 1. PL= 2, PAL = 1. PL = 3, PAL = 2)

At level 3 when choosing your pact if you choose pact of the chain instead of it's normal effects you can use a Sidekick of your level instead of one lower

Symbiosis

At level 6 Instead of a humanoid you're counted as a Devil. And in turn they are treated as Humanoid. You start to take on some psychical characteriscs of your patron. A lightly red skin, maybe darker eyes, or even a more evil laugh. Your body starts to become numb to certain conditions. Blistering heat and freezing cold feel alittle less so to you but more so to your patron. 

Each time you summon your patron choose fire or cold. You gain resistance to this damage and your patron loses their fire resistance.

Cold

Your patron's blood fills your veins and keeps you warm in the coldest of weathers.

Fire

Your skin becomes fire-resistant, as it takes on the properties of your patron

Mind Link

At level 10 your patron can use your spell-casting ability instead of their primary score while within 30 feet of you. When within 10 feet on your patrons turn it can expend your spell slot to shove a creature 15 feet way as a free action Your patron now lives in your head quite literally. Imposing part of themselves into your mind and taking part of yours in return. Your patron uses your dreams as their play place giving them a secluded place to torment souls. 

True Symbiosis

At level 14 the pact you've made with your patron defines the mark they place on your soul. Depending on what pact you have determines the feature.

Pact of the chain: Demonic chains pulse around your soul, reinforcing it. You and your patron gain a plus 2 to AC as long they are within 30 feet of you. Rattling chains can be heard when an attack misses because of this bonus.

Pact of the Blade: A Demonic forge burns around your soul. Any elemental damage dealt by you or your patron deal 2 extra damage die as long as you're within 30 feet of your patron. Brimstone and Sulfer can be smelled after your spells are casted no matter the effect.

Pact of the Tome: Your contract is written on your soul, giving you an understanding of magic no other has.  Your patron can cast your spells along with their own, although they use your spell slots.

PAct of Talismen: When your patron uses your talismen the die is upgraded by one.