What to call someone who always sincerely shows on the outside how and what he feels on the inside?

Solution 1:

A genuine person fits also.

free from pretense, affectation, or hypocrisy; sincere: a genuine person.

It can also be applied to personal attributes like feelings, sympathy etc.

Solution 2:

A transparent person may be a possible definition.

Transparent:

• easily understood; manifest; obvious.

• candid; frank; open.

Being transparent:

Like a looking glass into our soul. Often it is something that isn’t done. It is a way to keep our true self from being seen by others. Being transparent and showing your inner light to others is not something everyone does.

Solution 3:

Someone who does not hide their feelings is said to "Wear their heart on their sleeve", or for a single-word description, the person can be said to be "guileless", meaning there is no insincerity or pretense to them.

As defined in the Google Dictionary:

Guileless: "devoid of guile; innocent and without deception."