What exactly are yakshas? Is it good or bad to worship them? Will they get angry if you stop praying to them?
My point is that should we worship Kubera even if he is a yaksha or is he evil being that would want something in return like jinns,evil minded?
It is not that if you worship Kubera you will become evil minded or something just because he is a Yaksha.
He is one of the Dasha DikpAlas (ten guardians of directions) too and he guards the northern direction. Now, in all PujAs, worshiping the Dasha DikapAlas is almost mandatory. So, a pujA to Kubera can not be avoided.
And, individually too he is worshiped for gaining wealth as he is considered as the lord of wealth.
Now, if he is not worthy of worship at all, then the scriptures would not have suggested his worship.
For example, in Dashami Tithis, he is to be worshiped along with the other DikpAlas as per the VarAha PurAna.
Concerned about the population of the Universe and their mind-set of Directions so that orderliness should prevail to let them realise where they were, Lord Brahma devised a Plan. He created from His ears Six Devis viz. Purvi, Dakshina, Prathichi, Uttara, Urdhva, and Adhara- East, South,West, North, Above and Below respectively. The four other Devis thus created viz. Shobhaniya, Rupavati, Maha Bhagya Shalini and Gambhira Yukta represented the intermittent Four Directions. These virgins who were created requested Lord Brahma to name Lokpals also who would be the male counterparts and Brahma named them too viz. Indra, Agni, Yama, Nirruti, Varun, Vayu, Kubera and Eshana; Brahma Himself agreed to man Urdhva and the remaining Tenth Dasha (Direction) was spare. The Devi-Lokanayak equation was determined by Brahma. He also selected Dashami to observe the day for penance and eat only Dahi or curd to protect human beings from movement of Grahas, changes of Ayanasor Solar Progress and transformations of Ritus or Seasons and astrological and astronomical aberrations.
Also as per the same PurAna, Kubera the lord of wealth, can also be propitiated on the ekAdashi tithis for gaining wealth.
Brahma named the figure as Kubera and assigned him the task of protecting the material prosperity and possessions of all the Deities. He also made Kubera the Head of the Yaksha Clan to execute the assignment of Brahma. Also, worship of Kubera on Ekadashi day by cooking eats with the help of clean Agni- preferably by the person who eats himself after day-long fast would instantly make him eligible for wealth and also protection of wealth so earned.
So, since the worship of Kubera is not prohibited (on the contrary, recommended) in scriptures, you can very well do it without any apprehensions.
And, on the Dhana Trayodashi day (also known as Dhanteras) he is widely worshiped along with MahALakshmi for gaining wealth.
What exactly are yakshas?
Answered here: Who are Yaksha-s.
Is it good or bad to worship them? Will they get angry if you stop praying to them?
It's about personal liking. As mentioned in above link, Yaksha-s have attachive qualities like Raksha (also known as RAkshasa). Hence they are prayed in mode of passion (Rajas).
Upon stop worshipping, they will stop giving the results for whatever purpose were they being prayed.
My point is that should we worship Kubera?
Yaksha or Raksha necessarily don't mean "evil minded". They have attachive quality due to Rajas involved. Hence, those who have desires for anything, would worship in influence of Rajas.
If one acts desirous of wealth, then it's equivalent to worshipping Kubera.
BG 10.23 - ... among Yaksa-s and Raksha-s, I am lord of wealth (viz. Kubera) ...
Often people limit the worshipping only to "praying with hands folded and eyes closed". In reality, various worshipping ways are mentioned in Gita:
Whenever a person is desirous of wealth, he/she automatically worships Kubera, even if the person doesn't know the name. Similarly if person loves water bodies (river, ponds etc.), then he/she overall worships Varuna.
Whichever form one worships, the Self would fix one into that form. Though this way of worshipping is not methodical, it eventually leads to the supreme only.
BG 7.21 - Whichever form [of a deity], any devotee wants to worship with faith, I strengthen only that faith of them
BG 9.23 - O son of Kunti, Even those who being devoted to other deities and endowed with faith, worship [them], they also worship Me alone, following the wrong method.
Hence worshipping wealth (Kubera), eventually reaches to supreme. Eventually that would result in wealth. See this answer. Whatever attachive qualities come with money, one has to bear -- be it good or evil.
First of all, Lakshmi was a goddess of both wealth and prosperity. Kubera was the treasurer of the gods made by lord Shiva. Kubera take cares of wealth of gods and also provides wealth to his worshippers. In Bhagwad Gita, Lord Krishna had said that - among yakshas I'm the vittesha (god of vitta or finance) that is kubera. That's why he was also worshipped with goddesses Lakshmi.
Kuber is a guardian of gold and wealth hidden in earth.
Yakshinis or female yakshas are worshipped a lot to gain wealth. Yakshinis if worshipped properly with mantras and all, may appear in front of you. Yakshinis are well described in the book 'UDAMARESHWAR TANTRA'. If a Yakshini appears as a result of your worship, you can ask her to have relation with you as a mother, sister or wife. But it is always prescribed that one should treat a Yakshini as a mother only. If worshipped as a wife and if you are not a Tapaswi like Ravna a Yakshini has a right to destroy you if you spoil the loyalty with her. Also you have to satisfy her as a wife whenever she asks to.
Well the answer is getting diverted. ;)
It is not heard that Kuber worship is dangerous. So, you can worahip Kuber if you wish to.
