THe answer to the question.. who was the twelve apostle is simply Matthias.
There are quite a lot of apostles who were not mentioned of how they operated or what they did. The reason for this is quite obvious. If they were to keep to the gospel, then the left hand shouldn't know what the right hand was doing. Their goal wouldn't be to glorify themselves, but to spread the gospel. The book containing the story of Christ have no emphasis on the apostles but on Christ. Even Paul's books have no emphasis on what he was doing, but more of the things needed to be known. As for the book of acts, which was designed to tell the story of the acts of the apostles, mere intend was again so that people could understand in broad the work of an apostle, not that a lot of people understood it, but still the book was written.
There are 12 apostles for a reason. But it is not the reason most people think. Paul was not the only apostle, other than the 12. And when questioned about his apostleship he refered to it as though that of Peter and tried to drew a comparison. Not a comparison to put himself in the same level, but to the understanding of what is gift was. You see there are 12 differrent types of apostles. And in essension, if one didn't understand the function of or how to identify an apostle, it would be extremely hard to identify the office into which an apostle was appointed. Each apostle is send with a word. This word gives him power in times of need, when he struggles. No man can appoint an apostle, though there are those foolish enough to claim to be it. If God appointed one to be an apostle, you would be that from birth, just like if God appointed you, or Samaul, Moses, Jeremiah as a Prophet you would be that from birth. Or more Biblical correct:
Jer 1:5 Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations.
It is not something you choose, earn or achieve. It is within the very fabric of your soul. You cannot one day wake up and decide, I am going to be an apostle. You can however decide that you are going to be with Christ or not. Whether you are going to perform the duties of to which is expected of an apostle or not. Let me use a prophet as an example. The purpose and function of any prophet is to motivate, warn and edify. This they do with various methods, but ultimately listen to God. When the prophet see somebody is going to be in a car crash, he can warn that person, or he can not do his duty. What he has seen, he has seen. But he can decide not to fulfill his job. He can sense a spiritual attack on the body of Christ. He can do nothing, but he can also inform the specific target of the attack. Ultimately there are various different types of prophets. This is as clear as day when one perceive the various prophets in the Bible. Moses did not function even remotely the same as David, even though there was similarities which would identify both as being prophets and not something else. But they were both in the role of leading Isreal, yet they did it totally different based on the gift they received. The same it is was with the apostles. Now let's look at the scripture:
Act 1:22 Beginning from the baptism of John, unto that same day that he was taken up from us, must one be ordained to be a witness with us of his resurrection.
Act 1:23 And they appointed two, Joseph called Barsabas, who was surnamed Justus, and Matthias.
Act 1:24 And they prayed, and said, Thou, Lord, which knowest the hearts of all men, shew whether of these two thou hast chosen,
Act 1:25 That he may take part of this ministry and apostleship, from which Judas by transgression fell, that he might go to his own place.
Act 1:26 And they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles.
Two were appointed. These two had to meet two criteria... first they had to be an apostle and secondly they had to have been with them since the beginning of the Baptism of John until the end. The reason why there were only two out of two hundred, was because there was only two apostles that was with them from the beginning. For there were many others but they were not apostles and couldn't fulfill the function of the office of Judas. However only one of them were the same type as Judas were. Again born, not chosen by man, but chosen by God.
And God pointed out that Matthias had the same gift and placing and therefore he joined the twelve. And only once the twelve was completed and in union again could the next step take place. The Holy Spirit filled them and they could minister as their office held.
Paul was not the only other apostle, but he was well known for the reason that he was literate and wrote to the churches. He was however of the same type as Peter and understood this. He never considered himself as of high rank or value, but emphasised that he must be seen as the least of the apostles because of the persecution that he have done against the Christians.
I can think of a couple of the other apostles of the twelve that was also never mentioned or wrote anything. The reason is simple. It was not their purpose. It could even be that they were illiterate and couldn't write. The reason for them not being more evident is not for us to know. But understanding the gospel, these people would have tried to fulfill their obligation without trying to make a name for themselves to earn more in the Kingdom of God.