Who is the female warrior Maximou? Is it true that she never lost a battle? Was she a typical Amazon who lacked beauty? Read about her in a magazine which called her a relative of someone called Phillopapos. She supposedly appears in a medieval Greek literary work called Akritis.
The Acritic songs are a collection of poetry from the Byzantine Empire, written in medieval Greek and dating back to around the 9th century. They're very significant in the history of Greek literature - see for instance Chapter 2 in Kōnstantinos Dēmaras, A History of Modern Greek Literature, which is devoted to them and other works inspired by them such as The Chronicle of Moreas and the Achilleid.
Among the most famous of these epic poems is one called Digenes Akrites (Διγενῆς Ἀκρίτης, variously transliterated e.g. Digenis Akritas, Digenes Akritis, etc.) It's about the semi-legendary Byzantine hero Basileios, titled Digenes Akrites (roughly "mixed-blood border defender"):
Digenis Akritas, also called Digenis Akritas Basileios, Byzantine epic hero celebrated in folk ballads (Akritic ballads) and in an epic relating his parentage, boyhood adventures, manhood, and death. Based on historical events, the epic, a blend of Greek, Byzantine, and Asian motifs, originated in the 10th century and was further developed in the 12th century. It was recorded in several versions from the 12th to the 17th century, the oldest being a linguistic mixture of popular and literary language.
Digenis Akritas, the ideal medieval Greek hero, is a bold warrior of the Euphrates frontier, the son of a Saracen emir converted to Christianity by the daughter of a Byzantine general; he was a proficient warrior by the age of three and spent the rest of his life defending the Byzantine Empire from frontier invaders. The feeling for nature and strong family affections that permeate the epic anticipate the great Cretan national romance, Erotókritos (mid-17th century) by Vitséntzos Kornáros, and much modern Greek popular poetry.
Maximou, a female warrior, appears as a character in Digenes Akrites, in a rather unlikely passage in which the hero first defeats her in combat and then is seduced by her. From Adrienne Mayor, The Amazons: Lives and Legends of Warrior Women across the Ancient World:
One episode describes single combat between Digenes and an Amazon named Maximou ("Daughter of the Greatest"). She was "a descendant of the Amazon women brought back from among the Brahmins of India by the emperor Alexander." Leading a band of male rebel fighters, Maximou rides a milk-white horse with red mane, tail, and hooves (dyed with henna, a Persian custom brought to India). But for the duel she arrives on a black warhose (see chapter 11 for color choice of war horses). Wearing a green turban embroidered in gold and a breastplate over a tunic of purple silk, the Amazon carries a shield with an eagle device and an Arab spear and sword. They clash and fight strenuously. After Digenes kills Maximou's horse, they retreat to the woods where the Amazon removes her armor. Her "gossamer-thin silk shift reveals her lovely limbs and breasts", and the hero is smitten with lust. They have sex. In some versions Digenes later stalks and murders "the promiscuous creature" for seducing him. In this, Digenes seems to revert to the old Greek mythic script of killing the foreign Amazon. Some modern scholars believe that Maximou was originally the heroine of an earlier oral legend (perhaps about Amazons who returned with Alexander?), and that she was rather clumsily incorporated into this Christian moralizing epic.
I also found an entire article about Maximou and her role in Digenes Akrites, which you can read here: "Amazon Maximou ultimately failed in gambit for women’s privilege". You may not agree with its interpretation of the story (I don't), but it does provide a lot of direct (translated) quotes from the text, which might be useful if you want to see exactly what she said and did in the story.
All I can find about Philopappous is that he was another war leader and some kind of (probably older) kinsman to Maximou. You can see part of an exchange between him and Maximou in Elizabeth Jeffreys, ed., Digenis Akritis: The Grottaferrata and Escorial Versions, but unfortunately, there on Google Books, I at least can't view the pages to either side in order to get context.
To answer your specific questions about Maximou:
Is it true that she never lost a battle? No: she was defeated by Digenes Akrites in battle.
Was she a typical Amazon who lacked beauty? Again, by the text, no:
Maximou’s shift was gossamer-thin,
and it revealed her limbs as in a mirror
and her breasts rising just a little above her chest.
And my soul was wounded, for she was beautiful.
Of course, this is unlikely to be an accurate description of the real Maximou, if there was a real Maximou. But according to Digenes Akrites at least, she was beautiful.