Well, there is concrete evidence of the being of Khizr , although his name is not mentioned. The whole series of conversation is mentioned in the Quran. Read all of this:
[Shakir 18:65] Then they found one from among Our servants (Khizr) whom We had granted mercy from Us and whom We had taught knowledge from Ourselves.
[Shakir 18:66] Musa said to him: Shall I follow you on condition that you should teach me right knowledge of what you have been taught?
[Shakir 18:67] He said: Surely you cannot have patience with me
[Shakir 18:68] And how can you have patience in that of which you have not got a comprehensive knowledge?
[Shakir 18:69] He said: If Allah pleases, you will find me patient and I shall not disobey you in any matter.
[Shakir 18:70] He said: If you would follow me, then do not question me about any thing until I myself speak to you about it
[Shakir 18:71] So they went (their way) until when they embarked in the boat he made a hole in it. (Musa) said: Have you made a hole in it to drown its inmates? Certainly you have done a grievous thing.
[Shakir 18:72] He said: Did I not say that you will not be able to have patience with me?
[Shakir 18:73] He said: Blame me not for what I forgot, and do not constrain me to a difficult thing in my affair.
[Shakir 18:74] So they went on until, when they met a boy, he slew him. (Musa) said: Have you slain an innocent person otherwise than for manslaughter? Certainly you have done an evil thing.
[Shakir 18:75] He said: Did I not say to you that you will not be able to have patience with me?
[Shakir 18:76] He said: If I ask you about anything after this, keep me not in your company; indeed you shall have (then) found an excuse in my case.
[Shakir 18:77] So they went on until when they came to the people of a town, they asked them for food, but they refused to entertain them as guests. Then they found in it a wall which was on the point of falling, so he put it into a right state. (Musa) said: If you had pleased, you might certainly have taken a recompense for it.
[Shakir 18:78] He said: This shall be separation between me and you; now I will inform you of the significance of that with which you could not have patience.
[Shakir 18:79] As for the boat, it belonged to (some) poor men who worked on the river and I wished that I should damage it, and there was behind them a king who seized every boat by force.
[Shakir 18:80] And as for the boy, his parents were believers and we feared lest he should make disobedience and ingratitude to come upon them:
[Shakir 18:81] So we desired that their Lord might give them in his place one better than him in purity and nearer to having compassion.
[Shakir 18:82] And as for the wall, it belonged to two orphan boys in the city, and there was beneath it a treasure belonging to them, and their father was a righteous man; so your Lord desired that they should attain their maturity and take out their treasure, a mercy from your Lord, and I did not do it of my own accord. This is the significance of that with which you could not have patience.
Who, if not Khizr , is Musa talking to? Who is mentioned as 'Our Servant'? Even Sunnis Tafsir, Tafsir Ibn Kathir, recognizes these verses as referring to Khizr . Below is a link to Tafsir Ibn Kathir, where the story is mentioned:
The Story of Musa and Khizr
About Imam Mahdi (ajf), if Allah has the power to give Noah a very long life, he can give it to anyone He pleases. Hence, the statement that it is impossible for a person to live that long, is baseless, and said without proper understanding of the Quran. The age of Noah is mentioned as 950 years, and that too, life during the Prophet years. He must have lived even longer, i.e. the time before he received Prophethood.
And certainly We sent Nuh to his people, so he remained among them a thousand years save fifty years. And the deluge overtook them, while they were unjust. (29:14)
Some people might argue that since Noah was a Divinely appointed Prophet, other people can not be similar to him, or comparable to him. However, The Hanafi scholar, Sibt Ibn al-Jawzi, in his book "Tathkirat al-Khawas al-Ummah", pp 325-328 gives the names of 22 people believed by Muslims to have lived various ages from 3,000 down to 300 years! He just mentioned ages of ordinary Muslims, who were not Prophets.
Yusufali 36:8] We have put yokes round their necks right up to their chins, so that their heads are forced up (and they cannot see).
[Yusufali 36:9] And We have put a bar in front of them and a bar behind them, and further, We have covered them up; so that they cannot see.
No doubt shia will talk perpetually about how this individual justifies Imamate qualities, but one thing you need to understand is that most of sources on him that indeed can be utilized by shia is not reliable and is probably fairytales (i.e. Khidr found the fountain of youth so was able to live long)
Yes, he is no longer alive.
hope this helps
As for those who divide their religion and break up into sects, thou hast no part in them in the least: their affair is with Allah: He will in the end tell them the truth of all that they did. [6:159]
When Jesus found Unbelief on their part He said: "Who will be My helpers to (the work of) Allah?" Said the disciples: "We are Allah's helpers: We believe in Allah, and do thou bear witness that we are Muslims.[3:52]
Ibrahim was not a Jew nor a Christian but he was (an) upright (man), a Muslim, and he was not one of the polytheists. [3:67]
And who is better in speech than him who prayeth unto his Lord and doeth right, and saith: Lo! I am of those who are muslims (surrender unto Him). [41:33]
Nowhere in these verses, does it mention, sunni, shia, or any other sects.
For those who say, shia for life, sunni for life, take it where the sun don't shine.
Oh, here we go. You're gonna say he's still alive? Bring forth your proof, if you are truthful.