An Ascended Master is a Being, who through their own endeavours and hard work have paid all their karma, left the cycle of rebirth and attained Mastery over themselves.
An Ascended Master is a Being of Light who is not in body. An Ascended Masters has had many lives in a physical body, just like you – and through these lives and their own freewill and hard work have raised their vibration and consciousness to such a state as to have cleared all their karma and dross from their chakras and bodies. The Ascended Master is not the past life person, it is not the lower-self person who ascended, in truth the Ascended Master is the Higher Self of the person who strived to ascend. So when you call on Jesus, you are calling on the past life persona of the person who ascended to and surrendered to his Higher Self. It is not the persona of Jesus that responds rather it is the Higher Self of Jesus that responds.
We all have a Higher Self and all the Higher selves of those striving to ascend are Masters in waiting. All Higher Selves are great Beings who continually strive to become a Being of total purity and Light, they continually work on their own ascension process.
There are many Ascended Masters, some recent and others ancient, some who’s names we recognise and some names aren’t in our conscious awareness – some so old they are mythical to us now. Regardless of who they were in their countless past lives, they are returning now to help us in our own ascension process – for this is the time for many to ascend.
You can read more about some of them individually at this link - Meet the Ascended Masters
And you can see a list of many of these Ascended Masters, by no means an entire list as many are beyond my awareness, but these are the Masters I can verify as Masters. I have seen many of them in the Ascended Masters portal - Ascended Master List