We don't precisely know just how Tieria became to be so cold and distant from others. We know that he was created by the supercomputer Veda and raised by the paramilitary organization Celestial Being to be the perfect Gundam Meister. We know that he isn't human, and that he knows he isn't human. And we know that he considers himself responsible for the safety and security of not just the entire crew of the Ptolemaios, but of Veda and everything to do with Celestial Being.
Veda is the sticking point, here. It's clearly shown that Tieria takes failure extremely poorly - whether his own or other people's - and the first time Tieria makes what he considers a critical error on a mission he's left sobbing brokenly in his Gundam. Things really start to go badly wrong for him when he starts to notice that there are parts of Veda that he is being denied access to - parts that he previously thought were open to him. What starts off as a sickly suspicion coiling in his gut rapidly becomes a full-blown crisis. Due to the mounting problems surrounding Veda, the bridge crew of the Ptolemaios create a back-up OS to replace it should anything go wrong. Mid-battle, something does go wrong - the connection to Veda is completely shut down, leaving the Gundams without power, controls, anything. Fortunately, the back-up OS works, but that's only for 3 of the Gundams. Tieria's connection to Veda (that he thought made him oh so special) hampers the reboot of his Gundam, leaving him dead in the water. His comrade, Lockon Stratos, is forced to make a self-sacrificing leap in front of him. Lockon's machine is heavily damaged while trying to defend Tieria, resulting in Lockon being blinded in one eye.
The loss of Veda is not the only thing that's had a dramatic effect on Tieria's personality, however. Being exposed to "normal" life amongst the crew and his fellow Meisters has meant that slowly but surely, Tieria has come to certain realisations about humanity and about himself. He learns that it's okay to make mistakes, and to have more faith in other people. He even learns that he can come to his own conclusions and that he doesn't have to wait for Veda to tell him everything - this is clear in the way he follows Setsuna into battle against the Trinity siblings. It was not a mission or something specifically required of him, and yet because he felt that their actions were "wrong" of his own accord, he could fight them. He demands to be reprimanded for his disobedience after the fact, of course, and is mystified by the way Sumeragi (their tactical forecaster) shrugs it off. Tieria is present when Lockon finds out that Setsuna is indirectly responsible for the death of Lockon's parents, and sees the way that Lockon chooses not to hold Setsuna accountable for the sake of forgiveness, the success of the mission and humanity. Humans frequently confuse him - initially causing him irritation and aggravation at how unpredictable they can be - but eventually he becomes charmed by their ways, and strives to be more like them.
Where these two issues overlap, however, sits Lockon Stratos. It would be incorrect to say that he's the only person that's had a direct effect on Tieria's personality - after all, he grows much closer to Setsuna, Allelujah and all of the Ptolemaios crew throughout the course of the series. However, Tieria himself starts to center his opinion of the world around Lockon. It's Lockon that guesses that Tieria isn't human, it's Lockon that throws himself into harm's way to save Tieria when Veda abandons them, and it's Lockon that Tieria feels a crushing guilt over when he finds out that Lockon was hurt for his sake.
When Lockon then dies only days later, it's too much for Tieria to take. Even if he hadn't realised it himself, this was the man that Tieria has placed his faith in after Veda was lost to him. He lashes out at those around him before eventually deciding the fault is his own, and fights recklessly to avenge Lockon. His once uptight, entirely logical personality has been broken and reforged into something that's, yes, softer, but also wilder and more unpredictable as he struggles to come to terms with his emotions.
Revision!
Veda is the sticking point, here. It's clearly shown that Tieria takes failure extremely poorly - whether his own or other people's - and the first time Tieria makes what he considers a critical error on a mission he's left sobbing brokenly in his Gundam. Things really start to go badly wrong for him when he starts to notice that there are parts of Veda that he is being denied access to - parts that he previously thought were open to him. What starts off as a sickly suspicion coiling in his gut rapidly becomes a full-blown crisis. Due to the mounting problems surrounding Veda, the bridge crew of the Ptolemaios create a back-up OS to replace it should anything go wrong. Mid-battle, something does go wrong - the connection to Veda is completely shut down, leaving the Gundams without power, controls, anything. Fortunately, the back-up OS works, but that's only for 3 of the Gundams. Tieria's connection to Veda (that he thought made him oh so special) hampers the reboot of his Gundam, leaving him dead in the water. His comrade, Lockon Stratos, is forced to make a self-sacrificing leap in front of him. Lockon's machine is heavily damaged while trying to defend Tieria, resulting in Lockon being blinded in one eye.
The loss of Veda is not the only thing that's had a dramatic effect on Tieria's personality, however. Being exposed to "normal" life amongst the crew and his fellow Meisters has meant that slowly but surely, Tieria has come to certain realisations about humanity and about himself. He learns that it's okay to make mistakes, and to have more faith in other people. He even learns that he can come to his own conclusions and that he doesn't have to wait for Veda to tell him everything - this is clear in the way he follows Setsuna into battle against the Trinity siblings. It was not a mission or something specifically required of him, and yet because he felt that their actions were "wrong" of his own accord, he could fight them. He demands to be reprimanded for his disobedience after the fact, of course, and is mystified by the way Sumeragi (their tactical forecaster) shrugs it off. Tieria is present when Lockon finds out that Setsuna is indirectly responsible for the death of Lockon's parents, and sees the way that Lockon chooses not to hold Setsuna accountable for the sake of forgiveness, the success of the mission and humanity. Humans frequently confuse him - initially causing him irritation and aggravation at how unpredictable they can be - but eventually he becomes charmed by their ways, and strives to be more like them.
Where these two issues overlap, however, sits Lockon Stratos. It would be incorrect to say that he's the only person that's had a direct effect on Tieria's personality - after all, he grows much closer to Setsuna, Allelujah and all of the Ptolemaios crew throughout the course of the series. However, Tieria himself starts to center his opinion of the world around Lockon. It's Lockon that guesses that Tieria isn't human, it's Lockon that throws himself into harm's way to save Tieria when Veda abandons them, and it's Lockon that Tieria feels a crushing guilt over when he finds out that Lockon was hurt for his sake.
When Lockon then dies only days later, it's too much for Tieria to take. Even if he hadn't realised it himself, this was the man that Tieria has placed his faith in after Veda was lost to him. He lashes out at those around him before eventually deciding the fault is his own, and fights recklessly to avenge Lockon. His once uptight, entirely logical personality has been broken and reforged into something that's, yes, softer, but also wilder and more unpredictable as he struggles to come to terms with his emotions.