のは～の方（ほう）だ is an expression used to show that (A) (usually despite expectations) is (B). It can be translated as ‘the one that (A), is (B)’. This phrase is made up of the following parts:
のは - Used for nominalization (creating a noun phrase). は highlights contrast between two (or more) elements.
の方（ほう） - A combination of の, the 格助詞（かくじょし） (case marking particle), and 方（ほう）, a noun meaning ‘side’, or ‘direction’.
) - The 助動詞（じょどうし） (auxiliary verb) used for ‘determining’ something.
The literal meaning of this grammar point is something similar to ‘that which is (A), is on the side of (B)’. By saying that something is on the ‘side’ of (B), it comes across more gently than expressly stating that it ‘is (B)’.
To use this construction, attach のは to the end of a phrase, and then follow it with a noun + の方（ほう）だ. Note that のは will become なのは if the final word before のは is a noun.
I am the one that said it, so I will do something about it.
The one that broke the cup is Takeru! I have nothing to do with it!
This phrase is often used apologetically, to express that someone feels that they are the individual who is (more) responsible (for causing a certain outcome).