You've had so much experience fighting side by side with your companion the two of you have learned to anticipate each other's actions to such a degree that you may fight as a single entity. As a result you gain the following.
•Your two characters can fight on the same space and gain a +1 to attacks while doing so, as well as being unaffected by flanking enemies, as both characters can guard the other.
•When you are within 10 feet of your partner and they successfully make an attack on their turn, you can use your reaction to make a melee attack on that enemy, provided they are also within your range.
•When you are within 10 feet of each other and are both required to make a saving throw against an area effect–and only one of you passed–the other can add the passing partner's modifier being used in the throw in attempt to pass. This can be done four times per long rest.