Although I have left his endings almost to the very last, it is Klaus who is your 'natural' partner in Princess Debut in many ways. The Emerald Necklace displayed on the wall of your room in the Castle is his special Accessory. He is the only Prince for whom you actually will weep at the end of the day at the Amusement Park when you contemplate your impending separation. When you dance with him at the Ball of Saint-Lyon, the entire Kingdom will rejoice, including all of your erstwhile female rivals. In fact, there is no dramatic disclosure of your masquerade as there is if any other Prince is your partner. Apart from your option to accept or decline Klaus' offer of a Diamond Ring, the only choice you must make at the Ball is that of deciding the colour of sparkling apple cider you wish to imbibe in celebration.
Klaus may be the most desirable choice in terms of the Flower Kingdom but is it better to live the rest of your life concealing the fact that you are not the person every one believes you to be or is it better to be loved despite the fact that you are a 'fake'? All the other Princes are exposed to the truth and then declare that it is YOU they love, irrespective of your true status in life. Klaus never is given that option.
Klaus fell in love with the Princess years ago and is drawn to you because of that early connection. He refers to it again and again. In fact, you must reinforce those old memories for him in order to have the opportunity to make him your Partner.
Perhaps all that is required of you is a willingness to BE the Princess he loves.
The relationship with Klaus is complicated in another way. At the beginning of the game, the 'real' Princess warns you against ever touching the Emerald Necklace displayed on the wall. This leads one to suspect that Klaus is special to her. It is Klaus himself who later takes the Emerald Necklace from the wall to give to you at the Ball... but when you accept it, what effect will it have on the Princess?
I suspect that the reason you are free to love Klaus is because the real Princess actually prefers YOUR world to her own. She does not like Princes, nor does she care for dancing. In the Flower Kingdom, dancing apparently is a vital social activity. If you fall in love with Klaus, no doubt she will pursue his counterpart, Kyle, quite happily...
This would make the ending with Klaus truly a 'Happily Ever After' situation for every one in both worlds. The King never will realise that his daughter is missing and he appears to be unaware of any differences between you and the daughter he raised. In fact, the castle guard appears to know the real Princess better than her own father! Quite early in the game, the guard will remark that you appear to have become a very different person.
Magic is a wonderful device. If there are any unresolved issues, surely they will be erased through the effect of Magic...
Here then are both Happy Endings with Klaus: