What exactly is an echoist?

You could say, that an echoist is exactly the opposite of a narcissist. Echoists feel uncomfortable when attention is on them. They often are children of narcissists who have learned that being in the shadow of someone else and serving them is safer.  They often put everyone else first and tend to suffer from burn-out. Echoist also don't have a strong sense of self and self-worth.
Whilst an introvert might ask for help, an echoist tends to feel like a burden and believes that they have to do everything by themselves. The same with boundaries. Whilst an introvert might be ok with setting boundaries, an echoist tends not to set any. Their sense of inferiority stems from trauma and their behavior is a survival strategy.