His name is Hinto and he's a dustmare shipchild. 

I'm gonna give info cuz I wanna. 

Name - Hinto
Age - 19
Gender - male
Parents - Nightmare and Dust
Home - Nightmares Castle. 
Story -

Nightmare was using Dust for his sexual pleasure without the intent of making a child,  after a few months Dust felt really sick but none was in the castle at that time,  resulting in him giving 'birth' to Hinto alone. 
After a few hours Dust died from losing too much blood leaving Hinto into a puddle of blood and dust.  After Nightmare arrived he found Hinto and Dusts dusty jacket laying in the blood. 
After a few years,  at the age of 5 Hinto starts seeing Dust,  aka Phantom!Dust,  he begins to interact,  talk and sometimes play with P!Dust.  P!Dust isn't just a head with hands but whole body,  just like Cross Chara but cheerful as Mel from Ganz

Le full ref,  sorry I changed a few details but didn't like em.