Your Intelligence score increases by 2, and your Wisdom score increases by 1.

Demi Illithid do not have a definitive life span as Demi Illithid seem to either live as long as their host or a little longer. It's assumed the average Demi Illithid lives a little longer than the average human as most hosts are human.

Demi Illithid have a wider variety of heights and weights. Said height and weight will completely depend on what race the Demi Illithid was before their brain fused with the Illithid tadpole. Your size is medium.

Due to the attempted Illithid conversion you are considered an Aberration.

Your base walking speed is 30 feet.

You can see in dim light within 60 feet of yourself as if it were bright light, and in darkness as if it were dim light. You discern colors in that darkness only as shades of gray.

Due to the failure to completely convert into a full Mind Flayer Demi Illithids do not have mouths. Therefore you do not have the ability to speak vocally.

Due to the fusion between your mortal mind and will and the psionic power of the illithid tadpole the abilities of your mind have expanded. 
Telepathy
Without the assistance of magic you cannot speak. Instead you use telepathy to convey your thoughts. You have the magical ability to transmit your thoughts mentally to willing creatures within 120 feet of yourself. A contacted creature doesn’t need to share a language with you to understand your thoughts, but it must be able to understand at least one language. Your telepathic link to a creature is broken if you and the creature move more than 120 feet apart, if either of you is incapacitated, or if either of you mentally breaks the contact (no action required).
Mental Strength
You have resistance to Psychic damage and you have advantage on Intelligence and Wisdom saving throws.
Innate Psionics
Starting at Level 1 you have the innate ability to cast the Mage Hand cantrip at will.
At Level 5 you have the ability to cast the Disguise Self and Detect magic spells once per short rest each.
At Level 8 you can cast the spells Levitate and Detect Thoughts once per short rest.

At 8th level you tap into your metal powers. As an action you unleash a blast of psychic energy from your mind. All creatures in a 60 ft. cone must make an Intelligence saving throw or take 4d8 psychic damage. On a success a creature takes no damage at all. You can use this ability a number of times equal to your proficiency bonus and you regain all expended uses after a long rest.

Due to your lack of a mouth you cannot eat regular food. Instead you feed off of the psychic energy around you, which can also have an effect on your mood. (This is a non-action).

You can read and write Common, Deep Speech, and Undercommon.