Padre Pio in his letters speaks of the Guardian Angel: that's what he says

In a letter written by Padre Pio to Raffaelina Cerase on April 20, 1915, the Saint exalts the love of God who has given man such a great gift as the Guardian Angel:
«O Raffaelina, how consoled it is to know that you are always in the custody of a heavenly spirit, who does not abandon us even (admirable thing!) In the act that we give disgust to God! How sweet is this great truth for the believing soul! So who can fear the devout soul who studies to love Jesus, always having a distinguished warrior with him? Or was he not one of those many who together with the angel Saint Michael up there in the empyrean defended the honor of God against satan and against all the other rebellious spirits and finally reduced them to loss and bound them to hell?
Well, know that he is still powerful against Satan and his satellites, his charity has not failed, nor will he ever fail to defend us. Make a good habit of always thinking about him. There is a heavenly spirit close to us, who from the cradle to the grave never leaves us an instant, guides us, protects us like a friend, a brother, must always succeed in consoling us, especially in the hours that are saddest for us .
Know, O Raphael, that this good angel prays for you: he offers God all your good works that you do, your holy and pure desires. In the hours when you seem to be alone and abandoned, do not complain that you do not have a friendly soul, to whom you can open up and confide your pains to her: for heaven's sake, do not forget this invisible companion, always present to listen to you, always ready to console.
Or delicious intimacy, or blissful company! Or if all men knew how to understand and appreciate this great gift that God, in the excess of his love for man, assigned to us this heavenly spirit! You often remember his presence: you have to fix it with the eye of the soul; thank him, pray him. He is so delicate, so sensitive; respect it. Have constant fear of offending the purity of his gaze. Often invoke this guardian angel, this beneficial angel, often repeat the beautiful prayer: "Angel of God, who is my guardian, entrusted to you by the goodness of the heavenly Father, enlighten me, guard me, guide me now and always" (Ep. II, p. 403-404).

Below is an excerpt from an ecstasy that Padre Pio had in the convent of Venafro on November 29, 1911, in which the Saint speaks with his Guardian Angel:
"", Angel of God, my Angel ... aren't you in my custody? ... God gave you to me! Are you a creature? ... or are you a creature or are you a creator ... Are you a creator? No. So you are a creature and you have a law and you have to obey ... You have to be next to me, either you want it or you don't want it ... of course ... And he laughs ... what is there to laugh about? ... Tell me something ... you have to tell me ... who was here yesterday morning? ... and he laughs ... you have to tell me ... who he was? ... or the Reader or the Guardian ... well tell me ... was he their secretary? ... Well answer ... if you don't answer, I will say it was one of those other four ... And he laughs ... an Angel laughs! ... tell me then ... I won't leave you, until you have told me ... If not, I will I ask Jesus ... and then you feel it! ... I don't ask that Mommy, that Lady ... who looks at me grimly ... is there to do the demure! ... Jesus, isn't it true that your Mother is demure? ... And he laughs! ... So, young master (his guardian angel), tell me who he was ... And he doesn't answer ... he's there ... like a piece made on purpose ... I want to know ... one thing I asked you and I've been here for a long time ... Jesus, tell me You ... And it took so long to say it, sir! ... you made me talk so much! ... yes, the Reader, the Reader! ... well, my Angel, will you save him from the war that the rascal is preparing for him? will you save him? ... Jesus, tell me, and why allow it? ... you don't want to tell me? ... you will tell me ... if you don't appear anymore, well ... but if you come, I will have to tire you ... And that Mommy ... always with 1st corner of the eye ... I want to look you in the face ... you have to look at me well ... And he laughs ... and he turns his back on me ... yes yes laugh ... I know you love me ... but you have to look at me clearly.
Jesus, why don't you tell her to your Mother? ... but tell me, are you Jesus? ... say Jesus! ... Good! if you are Jesus, why does your Mommy look at me like that? ... I want to know! ... Jesus, when you come again, I have to ask you certain things ... you know them ... but for now I want to mention them ... That they were this morning those flames in the heart? ... if it wasn't Rogerio (P. Rogerio was a friar who was at the time in the convent of Venafro) who held me tight ... then the Reader too ... the heart wanted to escape ... who was he? ... maybe he wanted to go walking? ... another thing ... And that thirst? ... My God ... who was it? Tonight, when the Guardian and the Reader went, I drank the whole bottle and the thirst did not quench ... it owed me ... and it tormented me until Communion ... what was it? ... Listen, Mommy, it doesn't matter that you look at me like that .. I love more than all the creatures of earth and sky ... after Jesus, of course ... but I love you. Jesus, will that rascal come tonight? ... Well help those two who assist me, protect them, defend them ... I know, you're here ... but ... My angel, stay with me! Jesus one last thing ... get kissed ... Well! ... what sweetness in these wounds! ... They bleed ... but this Blood is sweet, it is sweet ... Jesus, sweetness ... Holy Host ... Love, Love that supports me, Love, to see you again! ... ».