Encryption through multiplication of encoded key
It is insecure in the sense that anybody with a calculator can reverse it (and a large set of people without one).

According to the Kerckhoff principle you should tell everybody the algorithm. So there is no security there.

After that an attacker will receive multiple ciphertext (ciphertext only attack). This attacker will quickly find a common divisor for the ciphertext. This common divisor is the key.

Furthermore, there are special values in your plaintext. In principle you don't want to have these in a cryptographic algorithm. Say for instance that you send a single space. In that case you would send bit string 00100000. As you can see there is only one bit set. Multiplication will result in a binary representation of the key "Hi", shifted 5 bits to the left. So an attacker can simply read the key straight from memory. If you can think of such a value, then your cipher is already considered broken.

Finally, a good cipher is a random permutation; a 1:1 mapping of the key to the ciphertext. That's the case in your algorithm, but the permutation is from $N$ to a set $key \times N$. Now you would have to encode this ciphertext efficiently or you end up with very large ciphertext values. This is why most cryptography uses modular arithmetic and XOR (which can be thought of as addition modulus 2).