To me. "Luigi... will follow...", as well as showing loyalty, also could show a bit of resignation to the fact that he follows, making it sad as well as sweet. Then there's the pleading or wistful "Take me with you!" and "I wanna go too!", the cry of "Wait for me!", and the most devastating plea of all, "Don't leave me!" - is Luigi afraid of being left behind, or perhaps, is he afraid of being left alone? The defensive "I'm not clumsy!" assertion also stands out to me, the question being is it meant to be indignant or dismayed? "Let me help!" could also be said so many different ways: like a demand (unlikely, imho), a gentle prompt (although the exclamation marks suggests no), a beseeching request (I think so), or even a desperate cry if Mario's particularly recalcitrant about receiving help. Things like "I fight ghosts!" and "I jump high!" are Luigi taking a little pride in what he can do - how he can measure up to Mario, although I wouldn't say it goes so far as to show some sibling rivalry, but rather, simply that this second Mario Bro. can stand on his own and has something to his own name. But like the "clumsy" bit, there's even insecurity underlying these outwardly boastful statements, especially when he follows up with "Don't fear ghosts..." - seems more like he's telling himself that, rather than giving Mario advice. Not to say that he's a coward, of course, because you've also got the gutsy statement "No one messes with my bro!" - perhaps that could be bravado, but I think it's a sincere flare of his fighting spirit. It's also interesting that while he seems to feel overshadowed by Mario, he certainly doesn't resent his brother for it, but on the contrary, holds him aloft on a pedestal with his incessant refrain of "Big bro!", and insistence to Mario that "[He's] so cool!". As I said before, he's certainly loyal and here, deep down, he's seemed to have accepted the fact that he's number two (hence the lack of rivalry in his statements, perhaps), which is even more apparent when you look at the full ending passage, rather than the "follow" bit alone: "Big bro... it's your choice... Luigi... will follow you... [I'm] here for you, bro. Always."