This is Manuel Laskaris Chatzikis who died in 1445 and was buried in the Pantanassa monastery at Mistra. The inscription on the fresco identifies him as "servant of the Despot, Constantine."
That is all we know about Manuel Laskaris Chatzikis, except that this image shows him wearing a style of hat that is otherwise only seen in portraits of John VIII, his brother Thomas, and their father, Manuel II. The hat is generally called a skiadion, but that is the wrong name and the right name is eminently forgettable.* Call it the Palaiologos hat, or the melon hat. Chatzikis also appears to be wearing a robe very like the one John wears in his portrait, although we can't be perfectly positive about the sleeves.
There was another Laskaris in John's ambit, about whom we also know nothing, except that John sent him to Italy to help make arrangements for the Council of Ferrara-Florence.
There were more Laskarids than just those two. They were a huge family, intermarried with all the other huge families, and you couldn't swing a cat in the Morea, or Constantinople either, without taking out half a dozen. But we are limiting attention just now to the coincidence of these two.
The Palaiologoi were accustomed to reward individuals who gave significant service with significant contributions to their wardrobe. Sphrantzes reports that Manuel II once gave him a dark-colored robe lined with fur and a chest that had belonged to his father, while John's wife, Sophia, gave him one of her green robes for his future wife.
On another occasion -- this when Sphrantzes was released nearly dead from imprisonment at Patras in a tower with mice and weevils -- Constantine gave him a double green tunic lined with fine green linen from Lucca, a red cap decorated with gold and a silken lining from Thessalonike, a heavy gold-colored caftan from Brusa, a green coat, and a finely worked sword. One might think green was the Palaiologos' special color.
We need more evidence for this, but it may not be too much to think that in this portrait of Manuel Laskaris Chatzikis we are seeing the emissary for Ferrara-Florence, and that he is wearing John's hat and robe, a signal to the cognoscenti that he was a person of high significance, not only to Constantine who ruled at Mistra, but also to the Emperor in Constantinople.
Late addition: The fact that Chatzikis had himself painted wearing this hat and robe does not mean that he actually wore them, but he might have been buried in them.
* For the sake of conscience, the name of this style of hat is kamelaukion.To my knowledge, this is not only the only non-imperial person shown wearing this hat, this is the only Greek fresco of this hat. All other images I know of this hat were painted in the West. I would be grateful for more information here.
** Go here for more information on the kamelaukion.