Unfortunately, he doesn't need a pet to help him guard a place. What he needs is a plant that he can take with him to the many worlds that he will invade in the future.
He also needs a plant that can eat anything that they encounter on those worlds. These have put the Blood Vine out of the running.
In order to make a good decision, he used Aries as an example and considered how well the three plants would do in Aries. He compared the pros and cons of all of them in Aries.
After thinking about it, he realized that he would not be able to take the blood vine there to provide him with the life force that could save his life in times of need. So the Blood Vine will be useless in Aries.
Even if he managed to bring the blood vine to Aries, he won't be able to feed the blood vine with his blood because of the lack of mana, and he won't be able to feed the blood vine with the blood of the Vulcans and Tundras either, because their blood is too hot and cold.
Even if the blood vine could feed on the blood of the Vulcans and Tundras on Aries, their bodies will be left to waste. Finally, the blood vine will not be able to eat races like the gem people, who have no blood.
These are things that the green face plant will have no problems with. He will be able to take the green face plant with him anywhere, and the green face plant will even be able to join him in fighting.
Finally, the green face plant will be able to eat any living thing, including the gem people. So even if there is no mana in the world he goes to, as long as there are living things there, he will always have life force, which he can use to cultivate or save his life in times of need.
He became even more sure of his decision when Pink Ivy told him that the green face plant can condense excess life force into crystal fruits instead of growing to become a large forest of vines like the blood vines when it has had too much to eat.
The green face plant creates the crystal life force fruits so that it can remain small and mobile. The fruits can also be stored for later use when it can't find prey and becomes hungry.
Arthur was very excited to hear about the existence of the crystal life force fruits because it meant that he could store life force outside of his Wood Vessel Sigil technique.
Pinl Ivy also mentioned that he could sell the crystal life force fruits to others or use them to raise other pets. She does similar things with her blood vine, but she usually needs to cut off pieces of the giant blood vine to sell them or feed them to pets.
These reasons made him determined to acquire the green face plant. So Pink Ivy helped him get one.
He had the options of getting a rank 1 green face plant, or rank 2, or rank 3, or rank 4, or even rank 5. Each one was more expensive than the last. They were about 10 times progressively more expensive.
He could also get a rank 1 version and raise it to rank 5 himself. Thanks to his cultivation of the Wood Vessel Sigil technique, he can help the rank 1 version quickly grow to rank 5 as long as he has a lot of life force to help it with.
He decided to get a rank 3 because he could afford it and could also raise to rank 4 with the amount of resources he had on hand. But she wanted contribution points, which he had a lot of, so he decided to get the rank 4 version.
Pinl Ivy went to Aries and served as the right commander of a division, but she failed to acquire enough contributions to get a nascent soul pill. So she was selling most of her stuff for contribution points, and she was selling them cheaply.
He, on the other hand, acquired a lot of contribution points because his division was instrumental in the ambush and defeat of the five gem kings that arrived on Aries as reinforcements.
He also earned a lot of contribution points when he fought the Sun King and assisted in apprehending him. So he could afford the low price that she was offering.
He earned 7,137 contribution points. The cost of a nascent soul pill is 10,000 contribution points. She only earned a little more than 3,000 contribution points.
She wanted him to pay 1,000 contribution points for the rank 4 green face plant. He thought that it was worth it to pay one-tenth of the value of something that could help her advance to the fifth mortal coil for a rank 4 green face plant.
After receiving the contribution points, she created a fresh rank 4 green face plant for him. She did this by upgrading a rank 3 one into a rank 4 one with her Wood Vessel Sigil technique.
She didn't have many rank 4 plants lying around because there was no need for them, and because the more powerful a plant is, the more troublesome it will be to take care of. So she only produced rank 4 plants based on the demand for them.
If he had paid her 10,000 contribution points, she would have upgraded the rank 4 green face plant into a rank 5 one for him. But if he had that kind of wealth, he would spend it on getting a nascent soul pill.
Once he acquired the rank 4 plant, he performed a bonding ritual with it. This bonding ritual will ensure that it won't try to eat him and will die if he dies. This was to ensure that the green face plant would try its best to save his life.