I have a theory, I do not really discuss them but I have an inkling of an idea. I have always had the idea that Tir and Riou were not only people that Luc respected, but people he also found as friends -- similar to how Futch is an acquaintance of Luc in the regards that they went through a lot together. I have a feeling that the reason that previous characters (with exception of Futch and Tuta) were not in the game as they would have been able to pull Luc back from his craziness, his issues with the rune and speak to him on a level which he would not only respect, but understand. The people he surrounded himself with were toxic, even though Sarah did love him -- she was toxic as she only did whatever he wanted, agreed with him and did everything in the name of love. Albert was only there to give advice on strats and Yuber just wanted to make a mess.
If anyone could have helped pull Luc from his dark period, it would have been Tir. The fact that they do not allow those characters to be present said a lot for the support system that he had. Riou and Tir would have stopped him long before he started to lose his mind, even aided him if possible. I always felt bad for Luc because he always appeared to have no one that could understand him other than his own brother, who barely understood him at all. I think in having two iconic people coming into the Grassland War would have stopped his movements forwards as he would have learned to better deal with his problems.
Watching Luc go from being tihs plucky punk to this misunderstood individual who had a lot of deep-seeded problems makes me feel that it would have obviously more than just Futch to make him see the light.
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