emilio's therapist says that emilio's personality is fixated at the earliest stage of development. in layman's terms, the therapist means that emilio's personality is at the earlier stage.

The therapist means that Emilio's personality is "stuck" at the earlier stage.

Fixation refers to a period that prompts a few ungainliness or awkwardness in the later life. This may prompt over the top tidiness, precision, persistence, irritability  etc. Every one of these practices indicate a few sorts of response development because of excessive fixation.