In regards to our Imams (pbut), the 12th Imam (pbut) is hidden by God, but he has sent his son and successor as messenger to all Muslims, he is also a messenger from Jesus to all Christians and messenger from Elijah to all Jews, and he is alive and physically present amongst us in this age and time, so we are not Shia Twelvers as we believe in the son and successor of the 12th Imam pbuh, which is Imam Ahmed al-Hasan.
Prophet Mohammed (pbuhap) at the night of his death in his Will mentioned that there will be 12 Imams and after them 12 Mahdis, which are also Imams, divinely appointed leaders. Here is the will recorded in 13 different Shia books:
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Will of the Prophet Muhammad pbuhap on the night of his death:
The Prophet Muhammad pbuhap said to Ali bin Abi Talib on the night of his death,
“O Father of Al-Hassan, bring me a pen and a paper”, and he dictated his will until he came to a position where he said, “O Ali, there will be twelve Imams after me and after them there will be twelve Mahdis. You, O Ali, are the first of the twelve Imams, Allah has named you in his heavens Ali Al- Mortatha, The Prince of the Believers, the Grand Truthful, the Bright Differentiator between truth and falsehood, the trusted, and the Mahdi (the rightly guided). These names may not to be truly attributed to other than you. O Ali, you are my successor over my family, their living and their deceased, and over my women. Whomever you affirm shall find me tomorrow, and whomever you reject I am innocent of her. I will not see her and she will not see me on the Day of Resurrection. And you are the successor (Khalifa) upon my nation after me. If death approaches you, hand it over to my son Al-Hassan the very beneficial. Then if death approaches him, let him hand it over to my son Al- Hussein, the martyr, the pure, the assassinated. Then if death approaches him, let him hand it over to his son, the master of the worshippers, Ali. Then if death approaches him, let him hand it over to his son, Muhammed Al-Baqir. Then if death approaches him, let him hand it over to his son, Jafar Al-Sadiq. Then if death approaches him, let him hand it over to his son, Musa Al-Kathum. Then if death approaches him, let him hand it over to his son, Ali Al-Retha. Then if death approaches him, let him hand it over to his son Muhammad, the trustworthy, the pious. Then if death approaches him, let him hand it over to his son Ali, the advisor. Then if death approaches him, let him hand it over to his son Al-Hassan, the virtuous. Then if death approaches him, let him hand it over to his son, Muhammad the Entrusted of the Holy Family of Muhammad peace be upon them. So these are the twelve Imams. Then after him, there will be twelve Mahdis. So if death approaches him, let him hand it over to his son, the first of the close ones, he has three names, one like mine and my father’s: Abdullah (servant of Allah), Ahmad and the third name is The Mahdi (rightly guided). He is the first of the believers.”
—Sheikh Al-Toosi, Al-Ghayba p.150
—Sheikh Hor Al-Amili, Ithbat Al-Hodat Vol. 1 p.549
—Sheikh Hor Al-Amili, Al-Iqath Min Al-Haj’a p.393-3
—Sheikh Hassan bin Soulayman Al Hilli, Mokhtasar Al Bassair p.159
—Al-Allama Al-Majlisii, Bihar Al-Anwar Vol. 53 p.147
—Al-Allama Al-Majlisii, Bihar Al-Anwar Vol. 36 p.260
—Sheikh Abd Allah Al-Bahrani, Al’awalim Vol. 3 p.236
—Al-Sayyed Hashim Al-Bahrani, Ghayat Al-Maram Vol. 1 p.370
—Al-Sayyed Hashim Al-Bahrani, Al-Insaf p.222
—Al-Fayth Al-Kachani, Nawadir Al-Akhbar p.294-9
—Sheikh Mirza Annouri, Annajm Al-Thaqib Vol. 2 p.71
—Al-Sayyed Muhammad Muhammad Sadiq Al-Sadir , Tarikh Ma Ba’d Al-Thohoor p.641-11
—Sheikh Al Mayanji, Makatib Arrassoul Vol. 2 p.96
—Sheikh Al-Korani, Mokhtasar Mo’jam Ahadith Al-Imam Al-Mahdi p.301-13
This is personal page of the Imam pbuh in which he communicates with the believers all over the world, and also the English version where his followers translate the messages.
https://www.facebook.com/Ahmed.Alhasan.10313/
https://www.facebook.com/AhmedAlhasanEnglish.10313/?fref=ts
This is "Biography of Biographies", some of the closest followers of Imam Ahmed al-Hasan pbuh talking about their journey to the Imam pbuh and various encounters proving that he is a real person which is alive now amongst us.
https://ahmedalhasan313.files.wordpress.com/2014/03/the_biography_of_biographies-20130908.pdf
Ahmed al-Hasan a.s. has written various books proving his claim through his divine knowledge and that he is truly Ahmed the Mahdi mentioned in Prophet Mohammed's Will, the son and successor of the 12th Imam- Imam al-Mahdi pbuh.
http://www.saviorofmankind.com/books/