Epileptic Trees: Ein is the only known Grim Angel who isn't leucistic, a condition where a person has unnaturally pale skin (even Malice and Nessiah, who are artificial and wingless respectively, have leucism). No one knows why. There are some guesses.
Goddamned Bats: Slime-type enemies, who can cause damage to your equipment with all of their attacks. This turns into Game Breaker/Game-Breaking Bug territory since it a) applies in practice mode and b) is incredibly awful when your equipment has single-digit use to begin with (and some equipment has a use of 1).
Macekre: Ein's Crowning Moment of Awesome speeches at the end of the game were heavily edited for the English version of the game in order to remove their controversial undertones.
Moe: Consider the genre. There's a lot of this to go around with the girls, but Ein in particular is adored, especially in Japan. "Ekukko" is the favored subject of a lot of fanboys' art pages...
If we only consider the core cast, the crown goes to Lina. Though Rose is a heavy contester.
Most Annoying Sound: Go through the Mireno Cemetery puzzle involving the ankhs and colored crystal alters that manipulate the water level in the subterranean lake. Now go through it without wanting to stab Ein in the face every time he chirps, "I can hear the sound of water!" Such a challenge is even tougher than the boss battle.
Scrappy Level: The Four Seasons Forest is infamous for causing player headaches. To wit: the forest requires you to walk in a specific sequence with changing seasons, but the whole thing is badly translated and thus even if you come up with the solution it still won't work. You'll almost inevitably have to consult a guide. Plus the whole area adds to your step counter, which means if you get lost, getting an S rank for speed is impossible and an A is at the least incredibly unlikely.
Ditto for the obligatory underground water ankh puzzle.
Tear Jerker: Ledah's death at the end of Chapter 6. Also watching Malice and the girl who likes Ein the most being sacrificed by Hector. Although the heroine gets better.
That One Boss: The Third Accursed, Archangel. Her Overdrive will most likely kill Ein in a single hit if he doesn't have enough status or protection and will cause good damage to the rest of the party and some of most annoying status effects. May the game have mercy on you if that affects the entire party. Oh, and she's all but immune to the Holy element, which is on most of your strongest weapons. Have fun!
Even with maximum possible levels, The Sage and The Fool in chapter 4 can cause a lot of players headaches.
Hector can easily become one if you rely on one element or he picks one he is immune to most your party's attacks. Kill him as fast as you can to avoid this problem.
Weirdly, the first boss you have to face, the phoenix Aghart. Not because it's particularly difficult or risky - it's impossible to die at this point in the game - except that Ein probably WILL get knocked out since you haven't been able to raise his skills yet. And Ledah only uses fire, Aghart is a fire beastie... this one's gonna go on for a while, before the story even gets started.
That One Sidequest: Oh, so you want the only available non-drop Rosier in the game with multiple uses? S-rank a Bomb Kid and never let any of them use their Overskills on you. (read: Suffer.)
Mention skipping stones to anybody who has played Riviera to completion, especially if they tried to play a high-score run. To quote one Let's Player:
Didja Redo: Hey, Heinrich Himmler called. He wants to know if you're offering cruelty lessons.
What an Idiot: Ein finds out that he was being brainwashed by his enemy so she can show him what he, and his allies, are doing to the good people.
You'd expect: Ein to attack the person who was previously his enemy and who had just gave him amnesia and accuse her of force feeding the stuff he saw. Instead: he believes everything she is saying and starts working against the people who not too long ago was his allies. True this works out for him but... Come on who in gods name would have listened to this person.