Heroes that can stall out the game and defend high ground easily can help get to a decently strong point with Arrival's passive. Keeper is also an extremely good ally in general for Crawler, which we will detail in this section.
The Crawler's nukes become extremely strong late game but magic or spell boosting abilities can boost them to absurd levels. Keeper once again is a good pairing because he can instantly refresh Dimensional Rip, and has Solar Bind.
Global Mobility Boosters:
Helps Crawler move around the map and farm or go to fights more effectively. These spells pair well with Arrival to give Crawler very significant global impact or the option to go to fountain and return fully healed.
AOE Crowd Control / Initiators:
Ideal heroes to set up Arrival. These heroes with a good entrance can deal a ton of damage. Also they are good heroes to teamfight with because Crawler before having Scepter has very limited mobility control.
Cores That Can Damage Through Spell Immunity:
Crawler is a carry that cannot damage through spell immunity, unlike almost all safelane carries. As such, it would be smart to draft Crawler with cores that can deal damage to Spell Immune enemies (usually physical but sometimes Pure). It is a reversal of a lot of usual position damage distribution.
Crawler is easy to kill for much of the game. These heroes can protect such a vulnerable hero very well especially early game. Counterinitiators count here too.
These heroes can improve key attributes for Crawler, allowing it to be a greater threat with it's abilities before it has farm and buildup of stats from the Arrival passive.
High Armor/Evasion/Attack Resistance/Physical Protection Heroes:
As a scaling magical nuker, Crawler is very effective against heroes who do not have protections from magic damage and focus on physical protection in all its forms.
Passive Reliant Heroes:
Crawler is naturally very effective against heroes that are reliant on their passives, even before it has Scepter due to making those passives weaker with Destabilize. Many of these heroes are also vulnerable to Blind/Disarm.
Kiteable Melee Heroes:
Later in the game, Crawler is much faster than these heroes and is very good at restricting them with it's various debuffs of Blind/Disarm and Slow/Root.
Heroes Vulnerable to Mana Loss:
Usually these are tanky heroes who still need to cast spells to do stuff, Spirit Touch can make them into non-heroes by depleting their mana in a similar vein to Anti-Mage so they would be alive but useless.
High Cooldown Heroes:
Heroes that rely on high cooldown spells to fight can face trouble. Crawler's fast respawn can potentially mean that in a normal game where these heroes would use their high cooldown spells to kill the enemy carry, it does not have that value here due to how fast the carry (Crawler) respawns. In principle it is similar to the normal way to counter these heroes (buyback) but without having to buyback.
Cosmic Crawler is one of those "all cores are getting BKB or we all die" kind of heroes due to its massive spell damage output and control with debuffs. The cores that are not able to use BKB very well will have a tough game against Crawler.
Crawler plays a favorable game against illusion heroes. Crawler deals a huge amount of AOE damage with its spells, and illusions will quickly fall under the Stellar Shift threshold and have permanent Blind/Disarm or Slow/Root. Arrival's damage in its higher levels also allows it to one-shot most illusions (the ones that take over 300% damage).
Heroes that Like to be At Low Health:
Due to how Cosmic Bind works, heroes that like to be at low health will find themselves continuously debuffed, especially by Blind/Disarm. This is particularly good against carries that rely on Satanic
Worst Enemies: (There are many of them due to how specialized Crawler's skillset is and how glaring it's weaknesses are)
Stat Thieves and Heroes that get stronger for each kill:
In general these are heroes that weaken Crawler or get stronger when they gank it or sometimes both. It's easy for these heroes to snowball to victory against Crawler who is ill suited to deal with them.
Due to Arrival's active giving a minimap indicator to all players when cast, these two can counter the global teleportation from global range. Silencer can use his ultimate as soon as he sees the spell being cast, and Zeus (with Scepter) can use his ultimate to find Crawler and Nimbus Crawler to stop it as well. As such these heroes can prevent Crawler from ever using ultimate until late game when it has low cooldown.
Crawler's spells are all active and silences make it into a creep. It cannot impact the game at all when silenced.
Gankers / Jump Heroes:
Crawler has no escape mechanisms so gankers who activate early can easily get a lot of kills on Crawler and ruin it's game. Late game Crawler is still weak to being jumped if it doesn't get to cast it's spells.
Base Armor Removal/Ignore:
Crawler's armor naturally almost all comes from Agility and the 6 base armor it has naturally and it is not well suited for a majority of the Armor items in the game. Heroes that can punch through base armor it will be able to tear apart a lot of tank Crawlers very easily into late game.
Heroes that in general can restore huge amounts of life will weaken Crawler, as Cosmic Bind needs heroes to be low on health before its debuffs activate. Fast healers can effectively remove Cosmic Bind debuffs for their allies.
These heroes will massively improve the heal-back of Arrival's expiration, allowing it to massively outstrip the initial damage it did and come back with a lot more health.
Damage Nullification and Reflect:
Arrival heals the full amount it's supposed to do at the end no matter how much damage it actually deals. As such enemies that nullify damage will end up with more health than before. Because Arrival is so slow and telegraphed, tanky heroes with Blade Mail can easily deal a huge amount of damage to Crawler with it. Nyx can do both.
As Crawler is the biggest caster core ever, heroes (and items) that counter INT cores are obviously big problems. Between the heavy mana costs/burns, magic resistance and scaling nuking that gets stronger, these heroes pose a massive threat in most phases of the game. The first 2 can be beaten with good use of upgraded debuffs (Silence and Root for Pugna; Break and Root for AM) and the Spell Immune heroes can be outscaled late game as they don't carry as hard as Crawler. Nyx basically cannot be neutralized as a threat and will be a massive threat to Crawler all game, especially since he can also block and return Arrival's damage. Mage Slayers will require good positioning to protect yourself from and good use of dispel and Black King Bar.
As stated above, Necrophos's healing and healing amp counters Arrival and Cosmic Bind, but the big issue is his ultimate counters Arrival's respawn reduction, which means that Necro can put you down for much longer for the enemy to make a play and can make the Crawler player much more likely to be forced to save buyback.