akumadeenglish:

Well, the whole flashback we’ve been reading since last August revolves around that very question – it all started with RC’s question to OC ‘Why did you deceive everyone and pretend to be me?” in ch130, 

so the reason will certainly be revealed within the flashback, most probably in this month’s chapter!

That said, one thing I can say for sure is that it wasn’t for practical reasons (as you pointed out, OC is the rightful “spare” of RC, so with both Vincent and RC being dead, he would have legitimately succeeded the title), but for emotional reasons!

I made this post a few months ago where I compared OC’s situation and motives for taking on his deceased twin’s identity with other famous manga series (Tsubasa Chronicle, Fushigi Yuugi and BASARA) and my conclusion was that in OC’s case it was probably a combination of

• guilt feelings (he felt guilty for surviving at the cost of RC’s death and wanted to erase his old “weak” self from this world)
• coping method (OC wanted to keep RC alive by pretending to be him) and
• inferiority complex / expectations of others (OC maby thought everyone would be happier if RC returned instead of him).

Furthermore, I remember there was an interesting theory in the JP fandom that the scene in ch95, where OC was confronted with his past in his nightmare, might actually be pretty much the answer to the question of “Why did OC take on his brother’s identity?”:

• Did you want me to forgive you?
  
• Did you want to break free from your weak self?
  
• Did you want to make everything yours?

OC denied all these accusations, but considering he is depicted as one big liar throughout the series and never really admits his true motives (e.g. JTR arc + Circus arc), I think there is a chance that all these statements were actually true to some extent!

• Did you want me to forgive you? 

→OC suffers from strong guilt feelings because he survived at the expense of RC’s death and believes it should have been RC who survived instead of him (ch91). 

Thus, he wanted to be forgiven by RC by taking on his name and carrying out the revenge on his behalf.

• Did you want to break free from your weak self?

→I think this is also a big part of the reason why OC decided to take on his brother’s name. He was a powerless, weak child who could do nothing but watch his brother being taken to the altar and killed before his very eyes. By taking on his brother’s name, maybe he wanted to erase his weak self from this world.

• Did you want to make everything yours?

→Lastly, this. While OC really loved RC and admired him, we also know that at the same time he’s always had an inferiority complex as well as some feelings of jealousy towards his healthy and sociable brother (ch131). 

As the second child in the family he literally had nothing, no inheritance, no watchdog title, no fiancee. So while this was certainly not the main motive, maybe a tiny tiny part of him thought that

if he took on his brother’s name, he’d be able to get what he had secretly desired. This would also tie in with what Sebastian said to OC in ch63:

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So yeah, it’s all just a theory but I personally thought it’s interesting particularly since we all know that Yana loves foreshadowing! xD

Anyway, let’s wait a few more days and hope that the next chapter will give us a somewhat clear answer to your question! 😀