Soulmate - depends on your definition really. I don't think I can answer this as I don't believe in the concept of soulmates.
Someone who totally understands you - impossible I reckon. Two people are so different that it's not realistic to expect somebody to understand you all the time. But love/a healthy relationship looks like somebody who is patient, who apologises when they get it wrong, who listens to your point of view, who either tries to understand you or just accepts you as you are without trying to work you out all the time. Understanding each other's communication style/attempts, and knowing what makes the other tick is pretty key, I think. But you don't have to know what's going on inside your partner's head all the time, and it's inevitable that you will clash occasionally, I don't think that means there is a lack of love or something wrong, as long as it's not happening all the time.
Is devoted to you - So going by the dictionary definition of "devote". "1. give all or most of one's time or resources to (a person or activity)." Hmm, at first I was going to say no but on second thoughts, I think fairly true. Especially in a family sense. Of course one should retain a sense of self within a relationship and individual activities are important, but if you look at something like work - it's possible for work to be something which supports the relationship/family (financially, by building a person's status in the community, gaining skills which contribute to family life, being admirable and respectable to your partner, etc) or something which drains from the relationship/family (a business which is losing money, working so many hours they have no time for the relationship, an extremely stressful job which has repercussions, work which makes the person so unhappy they are unpleasant to be around, etc). Of course in real life it's not as black and white as that, but yes the net result should be that most of a person's resources and time ought to be going into the relationship, with a smaller proportion of resources and/or time devoted to other people or for purely selfish pursuits. Which should also be a natural state for them, BTW, a natural choice which benefits them as well as you, a sense that what benefits both of you is aligned. If a relationship is totally draining a person, then they're not in a healthy relationship. So it needs to be that spending that time is both what the person wants and what benefits the relationship without martyring or thinking "Oh I should do this because it's best". If that makes sense.
And whose number one priority is making you happy - again, I think it could fall along the same lines. I was going to say that happiness is a bit of a flimsy priority when really an adult's priorities ought to be more substantial things like keeping food on the table, a roof over your heads, preventing illness or injury by following hygiene and safety, etc, but then I realised it's similar to my last point - that all of these things, food, childcare, financial support, cleaning, healthcare, do all contribute to a partner's happiness because to not pay attention to them would damage their happiness. So yes I think the family's or partner's happiness should be more of a priority than somebody outside the relationship's happiness, or their own happiness if it doesn't align. I was reading this thinking ooh, I don't think that I do particularly try to make DH happy very often, but then I realised that most of the things which are priorities for me because they make me happy or prevent unhappinss (looking after DS so that DS is happy/cared for, working to bring money and increase my own confidence etc, buying and cooking food that I like, making our living space look nice) also make DH happy/prevent his unhappiness and often I'll extend what I would naturally do for myself to cover him as well (washing his clothes as well as mine, tidying shared living spaces, cooking for him, etc) - nothing particularly romantic or groundbreaking or unexpected, just ordinary things but they do contribute to his happiness. And I'm not really thinking about these things, I just do them. Then there are the other unconscious happy-making things - supporting somebody emotionally, noticing when they've had a hard day and trying to do something for them, seeing something they would find funny so sending them a quick text, sex, saying welcome home and how was your day, asking after things they have told you, etc etc. Then there are conscious romantic gestures on top, which are rarer but still happen.
And just to add my own thoughts. For me love feels like safety and security. It's a safe place where I don't have to hide my real self or feel worried about being judged. Yes I occasionally have to watch my words or catch myself. It's about feeling appreciated and appreciating a person's contribution to my life, feeling that it's valuable, that I would be lacking something if he went away. It's about two people living life in a sort of Y shape, or perhaps two Ys, one upside down on top of the normal one. There's a part of life which is totally shared with shared dreams, shared wishes, shared happiness, shared priorities, and the work you both put into this part affects the other positively, and then you go apart and do your own thing which doesn't benefit the other directly but which makes you richer so you can bring even more life and newness into the relationship, and you can discover all of the life and newness that they are bringing too.