Who was the trustworthy person of the Prophet, sallallaahu alayhi wa sallam?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

If what you meant by the trustworthy person of the Prophet, sallallaahu ‘alayhi wa sallam, the person whom the Prophet, sallallaahu ‘alayhi wa sallam, entrusted with the names of the hypocrites of his, sallallaahu ‘alayhi wa sallam, time, then this was Huthayfah ibn Al-Yamaan . He was nicknamed 'the entrusted with secrets', as he was aware of secrets which no other person knew, as reported by Al-Bukhari.

However, if you meant the trustworthy person of the Prophet, sallallaahu ‘alayhi wa sallam, and the trustworthy person of this nation, then he was Abu 'Ubaydah ibn Al-Jarraah . Thaabit narrated that the people of Yemen came to the Prophet, sallallaahu ‘alayhi wa sallam, and Islam. He (Thaabit) said, "He took the hand of Abu 'Ubaydah ibn Al-Jarraah and said: ‘This is the trustworthy person of this nation.’” [Muslim]

An-Nawawi said, 'The trustworthy means the pleasing and truthful person. The scholars said, ‘He was not the only trustworthy person among the Companions but the Prophet, sallallaahu ‘alayhi wa sallam, attributed some characteristics to some Companions as they were distinguished by them from others. This means that it [this particular characteristic] was predominant in them [but this does not mean that others did not have it].”

Allaah Knows best.