The Time Lord named "The Nightguard" has had many incarnations each one of them different from the last.
The First Nightguard
The First Nightguard was an outgoing man with a sparky personality and a kind word for everyone. He encountered many enemies such as the Cybermen and the Terrileptils, but his greatest challenge was the promise he made to a dying Time Lord named "The Doctor." (The 10th Doctor) He had many Travels with many companions, however his Type 57 TARDIS was pulled into an alternate dimension where the majority of Earth's population was wiped out by a disease that kept the body living after death. While in this dimension he was bitten by one of these "Walkers" and slowly started to succumb to his wounds. Fortunately The Nightguard reached the safety of his TARDIS and began to regenerate in to his most infamous incarnation.
This incarnation of The Nightguard was a very shady character. Based on the information available this incarnation refused to be called the Nightguard. This was also around the time The Last Great Time War began. When the War ended with Gallifrey's "destruction" at the hands of the The Warrior (The War Doctor) he was beside himself in rage and grief. The War, having taken its toll on this incarnation and he collapsed due to exhaustion. when he came to he had a new face and new personality. And He was the Nightguard again.
The Second Nightguard
During this incarnation's aimless travels he decided to settle down on Earth disguised as an awkward high school student named Ben Paul, A name he decided would be permanent to cut (most of) his ties with the Time Lords. Now he faces a new Evil in the form of the Walkers. While fighting them he met many interesting people such as Lee Everett. In fending off the Walkers while trying to keep everyone alive, this incarnation fell off of a balcony and was impaled in the stomach by a pole, necessitating a regeneration. Gone was the tall, lanky 2nd Nightguard. In his place was the shorter more agile 3rd Nightguard.
The Third Nightguard
This Incarnation was perhaps the most egotistical Nightguard. He kept his compassion and love for humanity hidden behind a facade of uninterest and arrogance. This incarnation was forced to regenerate in orders to complete a mission of utmost importance.
The Fourth Nightguard
The 4th incarnation of the Nightguard was different than the others. He spoke in a Norwegian accent and occasionally used a gun. This incarnation regenerated as a result of the failure of the Omega-Sulu Project.
The Fifth Nightguard
The 5th Nightguard decided to begin the task that The Doctor gave him when he died. Find Gallifrey. Teaming up with an alternate version of The Doctor the two Time Lords eventually found it in a unknown sector of the Milky Way Galaxy's Delta Quadrant. The Nightguard finally got his revenge when the Daleks attacked. This incarnations regeneration came when he was shot in the back by a heavy weapons Dalek.
The Sixth Nightguard
The 6th Nightguard was a man of great intelligence and kindness. His predecessor's decision to send Gallifrey to the end of the Universe was enacted by this incarnation. He wandered around the Universe for some time, meeting up with old companions, and having a fantastic time. His regeneration came when he was hired as a security guard at a pizzeria which had been taken over by the Cybermen.
The Seventh Nightguard
The Seventh Nightguard kinda acted like he had a poll up his ass. After his predecessors regeneration damaged his TARDIS, he landed in 1975 in Gravity Falls, Oregon, where he met local con man Stanley Pines. He promised Pines that he would help him build a merchant fleet, and he delivered several ships to Pines including the aging RMS Titanic which ended up as the flagship of the fleet. This incarnation regenerated due to old age.
The Eighth Nightguard
The 8th Nightguard was much more fun loving than his predecessor. He enjoyed hugging people and was always willing to lend a helping hand. His regeneration almost never happened due to interference by the legendary Nightmare Child aka Bill Cipher. He was taken aboard the Titanic which was docked at Gravity Falls pier 47 and placed in its morgue. When he woke up he had a new face and no recollection of who he was.
The Ninth Nightguard
When he escaped from the Titanic's morgue, he instinctively went to the ships bridge. He was greeted by Stan's great nephew, Roderick "Dipper" Pines. Dipper was shocked to see an almost exact version of himself in adult form. This Nightguard disguised himself as Dipper and successfully defended Gravity Falls. This is his current incarnation.