The question you asked regarding the purpose of marriage and the characteristics of a spouse who could help one achieve this purpose, is an important issue that requires an elaborate and ample discussion. Certainly in this process having knowledge about the person's characteristics, morale, family situation, and much more can be very effective; however these aspects do not fit the scope of these pages. Upon studying this important and vital issue in the life of mankind, the Islamic scholars spend months and various sessions discussing and debating this issue. As you may already be aware, various books have been written about this topic. Thus, we only have the opportunity to discuss some general aspects of this topic in the following answer. However, it would be our utmost pleasure to assist you further for any ambiguities in this answer or any other specific point you would like more information on.
An eternal destination
Not only in marriage, but also in all other stages of life, one should have a major goal, namely to obey Allah (SWT) and to gain proximity towards Him. This is mainly because any other goal is transient and futile, and will eventually experience annihilation. However, if a person chooses to obey Allah (SWT) and lives his life according to the divine laws, the result of his acts will not be limited to only this world. Because his deeds have been interwoven with the eternal nature of Allah (SWT), he will benefit from the results of his deeds ever after. Moreover, such deeds bring him a step closer to Allah (SWT), the absolute perfection. The holy Quran repeatedly reminds us of this path: "All that is on earth will perish, but will abide (for ever) the Face of your Lord, full of Majesty, Bounty and Honor (1)", or "(Get the) color of Allah (Faith and Monotheism and Islam). And who can baptize better than Allah? (2)"…
Thinking and acting in this way might be a new experience for us, and might even sound strange and unachievable, but it is not impossible. Through following this method, a new world will appear to us, and perhaps we could view a scene of the paradise in this world.
Together, in the way of the Beloved
Since marriage is one of the most critical stages of the human life and plays a major role in both the physical and spiritual aspects of life, setting a goal in marriage has greater importance than many other matters in life. Islamic instructions have taught us to select obedience and proximity to Allah (SWT), as the goal of marriage. In practice this means to set the goal in life to help one another to serve and obey Allah (SWT); in such a life both would seek the satisfaction of Allah (SWT), and aim to perform His obligations in all instances of life as they help each other in this respect.
If our lives are built in this manner, the love and kindness towards our spouse increases every moment; as a result and based on what was mentioned before, such a life will not be transient. Even the Infallible Imams (PBUT) have stated that such a union will not be broken even in heaven; such a spouse will be more glorious than Hur al-Ein(3).
A spouse like this…
Your second question can also be answered based on what was said above. For a spouse to be able to help his/her mate in this way, he/she should have also been able to freely and consciously select such a goal, out of all other possible goals that different people choose for their lives. The significant feature of these individuals is their sincere efforts in piety and restraint from the unlawful acts that Allah (SWT) has forbade.
But how could we practically examine such characteristics in our future spouse? We have to pay attention to the fact that the family of our future spouse plays a significant role in his/her morale and mentality. It is advantageous if within the family, organization and mutual understanding exists. In other words, the father aside from his closeness to the family is considered a powerful manager, and the mother protects and remains in her kind and loving position during the difficulties and shortcomings of life.
Based on this aspect and apart from the effects of society, the main structure of the boy's character is similar to that of his father's character, and the character of the girl is similar to that of her mother's character. Therefore, one can get a feel of his/her spouse's character and goals by investigating his/her parent's character.
Another way of knowing your future spouse is to research about the person and family from the friends and acquaintances of that family.
Besides all these, it is noteworthy to mention that beauty and elegance is important, but should be considered in the second stage of this process. This is mainly because one has to deal more with the morality and character of his/her spouse than his/her beauty or even academic degree. We can easily learn this fact if we research our surroundings; how often we may find that despite the fact that many couples have high academic degrees and or special qualities, they still experience a dark and strained marriage life.
Hence, there remains no doubt that the depth of love and its endurance depends on our intention in marriage. So the deepest goals make the strongest relations. If both sides set a divine goal for themselves, it is obvious that they will do their best to move towards their goal and help each other along the way. Love and kindness is definitely deeper and stronger in such a life.
The most beautiful union, the deepest love
A good example of this beautiful and affectionate life is the life of Amir al-Mu'minin and Lady Fatimah (PBUT). As a testimony to what was previously discussed, it is worth mentioning a short scene from their life; a life that its main cause of unity was the love for Allah (SWT) and the consistent goal in life was to seek His satisfaction. Nothing else would interfere with this goal.
Once the Prophet Muhammad (PBUH&HP) visited Amir al-Mu'minin (PBUH) and Lady Fatimah (PBUH) after their marriage. The holy Prophet asked Amir al-mu'min: "How do you find Fatimah?" Amir al-Mu'minin (PBUH) replied: "She is a good helper in obeying Allah."
The Holy Prophet then asked Lady Fatimah (PBUH) about Amir al-Mu'minin (PBUH). She said: "He is the best husband." After hearing this, Prophet (PBUH&HP) raised his hands and prayed: "O Allah! Unite them and join their hearts; make them and their progeny the heirs of heaven; honor them with a purified and blessed progeny. Bestow abundant blessings upon their progeny and appoint them as Imams who guide people to obey you and command based on your satisfaction."(4)
It is also remarkable that they did not put aside their normal life and solely stick to worshiping, but they used to live normally as other spouses and parents do. However, their mere purpose of eating, sleeping, working, and all other efforts in life, was to be able to obey Allah (SWT) and achieve His satisfaction; they only saw Him and thought only of Him. Thus, the secret behind their deep love for each other was this. This is the satisfaction of Allah (SWT) that unites the strongest bonds among beings.
Considering carefully, it is interesting that although Lady Fatimah (PBUH) had numerous virtues and accomplishments, when Amir al-Mu'minin (PBUH) describes her, he states "her assistance in the way of obeying Allah (SWT)" as her highest virtue.
Marriage, peace and tranquility in all respects
By having such an attitude towards marriage, the practical result that the holy Quran states for marriage would be acquired completely: peace and tranquility in all aspects of life. As you may already be aware, in the 30th chapter of the holy Quran, Allah (SWT) introduces tranquility and peace as the purpose of marriage:
"And among His Signs is this, that He created for you mates from among yourselves, that you may dwell in tranquility with them, and He has put love and mercy between your (hearts): verily in that are Signs for those who reflect."(5)
Since Allah (SWT) has not specified any constraints or limitations for this tranquility, it is understood that the tranquility applies to all aspects of life. In other words, after marriage, the two spouses will achieve tranquility in terms of financial, social, physical, and most important of all the spiritual aspects; their agitations and unrest would turn into peace and tranquility.
Naturally one of the motives in marriage is achieving such peace and tranquility, for everyone finds some sort of agitation and impatience within him/herself before marriage. However, if one seeks to obey Allah (SWT) and gain closeness to Him, he will achieve deeper tranquility and peace than all the others. As mentioned by the holy Quran, what really quenches the thirst of one's nature is obeying and remembering Allah (SWT).(6)
A fine start, an introduction to the path
In spite of all these facts, it should be noted that selecting an appropriate spouse, with all its importance, is only half of the way. In fact, selecting a proper spouse does not suffice; one also has to live proper and stand beside him/her in all the incidents of life. Paying attention to this fact has an important role in bringing happiness and prosperity to the life. Therefore, let us chose a divine goal for ourselves and live as He wishes in every moment of our lives.