Summoning five star characters in Fire Emblem Heroes's horny casino is rare. While you can promote weaker characters, the mechanic is a terrible grind.
Characters in Fire Emblem Heroes are ranked from one to five stars, with a higher number of stars meaning a higher potential for good stats as you level them up. While you can play with heroes of any star ranking, four and five star characters are just demonstrably better in play. There is a method for raising the star level on character, but it just sucks.
You can use hero feathers for "Unlocking Potential," which raises the rating of a hero if they're level 20 or over. Raising a three star to a four star is expensive, but not prohibitive -- it costs around 2,000 feathers. Raising a four star to a five, however, costs a whopping 20,000 feathers, as well as 20 amulets in the same colour as the hero type. The amulets you can get from training maps, but you're just not going to see that many feathers any time soon.
You earn a few free feathers every day from your friends, and can win much more at Arena Battles. However, the maximum reward for the Arena is 1,600 feathers. There's no way to purchase feathers, and you only get three Arena duels a day unless you buy an item. Even then, the feathers are only rewarded after the end of the 6 day duelling session. In order to raise the rank of a three starred hero, you'd be waiting about two weeks.
For a four star hero, that wait time jumps to close to three months. People are so eager to earn more feathers that Nintendo's tweet promotion for Fire Emblem Heroes, which would reward players with 10,000 feathers if the tweet reached 10,000 retweets, was launched last night and met this morning. It currently sits at over 35,000 retweets.
— Nintendo of America (@NintendoAmerica) February 7, 2017
There is a way to lower the cost of a promoting a hero. Merging two of the same heroes together will give one hero a stat boost, and lower the feather cost by the feathers you'd earn for discarding the other. You can get up to 1,000 feathers for discarding a four star unit, but that's for five star heroes you're unlikely to discard. Three star units will net you 150 feathers, and four stars give 300 feathers, and that barely makes a dent in the cost for promotion. Even if you are somehow discarding or merging four star heroes all the time, you'd need to throw away 20 heroes to account for the cost of promoting one hero to five stars. Out my current roster of 34 heroes, 12 of them are four stars.
Furthermore, when you promote a hero that removes whatever stat bonuses you've received from merging, and puts them back down at level one. It just seems unlikely that a level four hero that's been merged multiple times is worth promoting to five stars, especially if they have been levelled up to over level 20.
Other character collecting games with this kind of levelling mechanic have the same kind of frustrating grind, but I've never seen it laid out quite so pointlessly. In Love Live you can turn cards into more powerful versions of themselves if you have two of the same cards, and max out their "bond," stat by using them in play enough times. While it can be difficult to get a second copy of very rare or event only cards, it feels a little more fair than dangling the option in front of you but making the requirements absurdly high.
You're probably better off playing Fire Emblem Heroes ignoring the ability to promote characters entirely. Given the high cost, the difficulty in obtaining the currency, and the fact that you have to grind to level 20 just to be able to do it, it makes more sense to either use the heroes you've got or summon some more. You've probably got a better chance at that five star Donnel by rolling the dice.