Eriol's personality is more complex than it looks like. There are layers and layers to explore, and sides he'd only show to some people.
First of all, we have to keep in mind Eriol's nature. He comes from Clow, he keeps Clow's memories. Therefore, a large part of his desires, flaws, and virtues will be exactly like his past life's. However, there's a huge problem: How was Clow really like? How much of Eriol is Clow's? As a dead character that has only gotten a couple of flashbacks and mentions in other CLAMP canons, it's extremely hard to pin him. However, I don't believe Eriol is Clow II. He's not even Clow as a whole, just a half. After so much time, some part of him has to be different from the original fragment of soul. But which one? And what part of Clow did Eriol kept in his half? No way to tell; we can only suppose.
So... I guess I'll just be generic and try not to classify traits into "clowish" and "non-clowish".
The façade is the part of Eriol most people, especially classmates, interact more with. He looks like the kid next door: kind, polite, smart, wise, and talented in several fields (sports, academics, manual work, cooking, sewing, arts, even music in anime). Also dorky, sometimes, letting his creativity out on silly things like supporting Yamazaki's lies. Socially speaking, a tad distant -maybe because he's somehow old-fashioned British after all-, but he'll happily join his friends and his friends' friends when they invite him over. He'll aid you when you're in a pinch. He'll always dedicate you a warm smile.
But what about the mysterious, creepy grin he wears when people aren't looking? What is he hiding? Should we fear him?
Naaaah. That's just his other self. Potentially more dangerous, yes, but not really. Eriol IS creepy, and seems way too happy when he creates all those situations that'll make Sakura and the rest sweat. It's probably strange from an outsider's point of view, and it's so easy to label him as a sadistic bastard. But see, he smirks in such a fashion because he's having fun by doing all that magic and creating chaos, not because he's looking forward to make people suffer. In fact, he doesn't like that at all. He can only enjoy himself since he knows beforehand everything will turn out fine and nobody can get (too) hurt. I'll explore some more of this in the misconceptions section.
Deep down, apart from these façades (perfect guy vs. creep), there's the part of Eriol only his guardians and people who look close enough can tell.
One: he thinks he had control and he takes future for granted. This is due to his clairvoyance powers and everything else he inherited from Clow. When you can see so much ahead, you really think a) it's something fixed and not mutable, and b) you can lead things into happening the way you want them to, because you saw them. I think this is one of these "clowish" things, as it was Clow who had everything planned ahead -even in other dimensions. Eriol just grabbed this point of view and way to do things, and tagged along. In consequence, when something unexpected happens -particularly related to hearts, so "hard to predict"-, it comes to him as a surprise. Thankfully, he loves surprises.
Two: he's, in fact, kinder than he looks like. Three persons noticed this. There's Sakura, who probably picked this up since he reminds her of her father, who is a sweetheart. So it's more of a gut feeling, but she had a point. There's Tomoyo, who observed that Eriol looks at Sakura, and sometimes at Li, with gentle, protective eyes; therefore, she thought he was kind. There's Kaho, who tells him right to his face; he replies she might be the only one who truly thinks that. Considering she knows him and his reasoning tons (TONS) more intimately than the other two, it makes it significant for him.
But actions have their weight. Beyond the mischief, he truly cares about Sakura, the cards, Kero, and Yue's welfare. He wants them all to be happy. He wants the cards to change and survive. He wants the guardians to get used to their new life with their new Mistress. He even gives his "descendant" Li, who hates him, cryptic advices on how to make improvements with Sakura. But then, we see he's rather sweet towards his closest ones, not only the main characters. He might've created his guardians because he needed backup for the last battle, but he also wanted to make Yue understand he had his own life different to Clow's. And yet, he never treats Spinel and Ruby like tools. Hell, he thinks of them as his family! In the manga, he even prepares lunch for Nakuru everyday and yummy food in general, and he sewed her a yukata for a festival.
Three: he's so mysterious and secretive he never tells what he really thinks. Ruby Moon had absolutely no idea what her Master's plans were or what they were doing in Tomoeda. Spinel Sun seemed to have a more correct, general idea, but we have to wonder if it's because he's the most empathic in the set of guardians and can read Eriol better than his sibling, or if Eriol confides him and trusts him more. I'd like to think it's the former, since Spinel is so clever. It could be anything, though.
Four: and then there's the sense of humor. Well, truth to be told, most of the silly stuff we associate with Eriol comes from his anime characterization; manga!Eriol only kept his fondness for Yamazaki's stories, and the obsession on plushie offensive forces. While I think manga!Eriol does have a sense of humor, it's not as transparent and over-the-place as anime!Eriol. Anime!Eriol is more of a trickster figure and takes everything less serious; then again, the reason manga!Eriol has to angst over doesn't exist in anime. Also, it is implied in the Wonderland episode that he took it from Clow. However, I don't think that affirmation really contradicts manga canon. Because in the book Soel and Larg: The adventures of Mokona Modoki, which is CLAMP canon, Clow pulls a couple of pranks.
Five: he's, above all, shy. He can look outspoken, but he really isn't. Then again, he has too much to hide to be comfortable with people, and too introverted to admit he cares for someone.
Six: he's wise. Then again, with all the accumulated years: centuries of Clow's reminiscences plus the years he's lived as himself... That's a lot of knowledge and experience. And yet, with all that power and wit, he's still down to earth. Actually, he doesn't even want that much power...