a riddle i places b4 u

$do$ $u$ $know$ $y$ $i$ $exists?$
$bcos$ $i$ $is$ $that$ $that$ $does$ $not$ $exist$
$in$ $plane$ $terms$ $i$ $wanders:$ $y$ $is$ $i$

$sum1$ $and$ $i$ $in$ $a$ $relationship:$ $its$ $complicated$
$at$ $times$ $i$ $is$ $in$ $heaven$ $with$ $an$ $angle$
$\&$ $other$ $times$ $on$ $the$ $ground$ $-$ $bless$ $i,$ $4$ $i$ $has$ $sin$

Answer

I'll take a go at it and say it's

The Imaginary Unit.

my reasoning:

do u know y i exists? bcos i is that that does not exist

i is something that does not exist

in plane terms i wanders : y is i

This seems like something related to plotting on a coordinate plane: In the complex plane, the y-axis is the imaginary axis representing i -- the imaginary unit.

sum1 and i in a relationship: it's complicated

Now it definitely is a mathematical quantity. and when we add 1 + i we get something "complex": a complex number! so we can confirm that i is the imaginary unit.

at times i is in heaven with an angle

i can be in the exponent of e with an angle, say $\theta$ to get $e^{i\theta}$

&& other times on the ground −− bless i, 4 i has sin

i appears on the ground when we use Euler's formula to rewrite $e^{i\theta}$ as $cos\theta + isin\theta$