Right now, as there are only 2 major characters introduced, just those two, plus toolbox who you know next to nothing about. I will update this page after every chapter. Tell me if I missed something. Mild spoilers, but most of what's in here is either mentioned or is directly guessable, or is unrelated to any plot twists in the future.

Slime

Name: Unknown

Race: Unknown type of slime

Level: Unknown

Notable abilities:

Changing the appearance and properties of his body with some restrictions on mass, volume, properties, and density of the body. Of course, to increase mass, equivalent mass must be ingested. To reduce mass, it is simply released as green slime matter.

Restrictions: Unknown

Weaknesses: Unknown

Maximum size shown (yet): Dragon

Minimum size shown (yet): Spider

Relationships:

Unknown relationship with Gobsuke

Allies: Arachne (Partner)

Spider

Name: Arachne

Race: Unknown type of spider

Level: Unknown

Notable abilities: 

Ability to eat a lot (Gluttony).

Did you know this story is from Royal Road? Read the official version for free and support the author.

Ability to convert all eaten mass into thread with up to 90% efficiency.

Ability to manipulate and change the properties of said thread with little to no restrictions.

Weaknesses:

Small size

Weak body

Others unknown

Low defences

Relationships:

Unknown relationship with Gobsuke

Allies: Slime (Partner)

Toolbox

Existence (Is he even real): Unknown

Name: Unknown

Race: Human

Level: 1

Notable abilities: Unknown

Weaknesses:

Low level

Low combat potential

Weak body

Others unknown

Relationships:

Unknown

Gobsuke

Name: Gobsuke

Race: Green Goblin

Level: 1

Notable abilities:

Can summon any goblin once a week

Weaknesses:

Low level

Low combat potential

Weak body

Weak race

Others unknown

Relationships:

Unknown relation with the slime.

Unknown relation with Arachne.

I'm sorry that the main character still has not shown up. He's still got some time.